The series is rather nondescript about this stuff, and I did not find any recipe that is really described as 'instant pottage'.* As such I preliminarily infer that this is a humorous but not totally inadequate but interpretative description by Jones and his writers.
For recipes and practices from medieval cookery in England that approach such an imaginative portrayal of a type of pottage, we might benefit from looking at likely candidates for such a strange stew:
This could be anything, just as any ordinary pottage. The only thing to observe is that this is pre-prepared — 'convenience'? — food that just underwent a cycle of wetting (cooking) and drying, only to be made into a soup again.
What exactly is pottage?
pottage the medieval term for a semi-liquid cooked dish, typically based on cereal, which in various forms was a mainstay of diet for many centuries.
The word comes from the French potage meaning something cooked in a pot. It thus has a very wide application. It is no longer in use in English, its function having been largely taken over by porridge, which is the same word, slightly changed and now having a more restricted meaning. […]
In the Middle Ages, and especially in Britain, pottages were eaten by all, from the poorest to the richest. The simplest kinds were cereal pottages: oatmeal in the north, barley, rye, or wheat frumenty in the south. To the rich these dishes were an accompaniment to meat; to the poor they were complete meals. Pease pottage, made from dried peas, and other pulse pottages were equally important. These dishes might be quite plain or contain herbs or other additions. Vegetable and meat pottages were thickened with breadcrumbs, oatmeal, eggs, or, for the rich, amidon (wheat starch). Most contained onions. One universal favourite was green porray, made from a large number of green vegetables and herbs which were boiled until thoroughly soft.
— Alan Davidson, Tom Jaine (eds): "The Oxford Companion to Food", Oxford University Press: Oxford, New York, 2014.
As such we observe that the base of common pottage is usually grain or pulse, often combined to give better flavour and enhanced nutrition. As we now know amino-acids from grains (like barley, oats, rye and wheat) combined with legumes (such as peas, beans etc) balance out each other out: deficiencies in essential amino acids in one staple come from the other ingredient.
As such, two basic ingredients to either use in isolation or especially combined would be
To make Amydon. Recipe whete & stepe it ix dayes, & change þe water every day twyes; than bray it in a morter right small, & clens it throgh a haryn syve, & lat it stonde tyll it be sett; þen put onto þe morter & bray it in a clothe to it be dry.
— To Make Amydon
- canebyns, or frizzled beans
The beans were soaked until they began to swell and burst through the hulls. Then they were dried again, the hulls discarded, and the beans chopped and toasted before being stored away. The beans kept better, and the flavor of the pottage was increased with the toasting.
— Victoria R. Rumble: "Soup Through The Ages. A Culinary History with Period Recipes", McFarland: Jefferson, London, 2009.
Amydon or amidon is often defined as for wheat starch only. But this procedure is the same for any type of cereal grain or plant material. It has to be noted that the resulting product should be reasonable easy to dry out and should not contain too much 'good fat' (unsaturated). For example, grounds from oats keep well the least, because of their high fat content that will go rancid. Such a procedure has to either remove the fat and really concentrate the starch alone or be eaten rather swiftly after making compared to the other grains available.
To such a base of relatively dry material you add a liquid and have a not so bad base for some kind of a cold soup.
Add any ingredient you like. These could be herbs and spices and anything that may also dry out well for the 'instant' powder. Or when preparing the actual dish, add Fresh herbs, nuts, leaves, roots, tubers or fruit found in woods and meadows, as all those make a nice addition. If you also catch or carry animal ingredients (like milk, cheese, eggs, meat, fish) the taste and nutritional value might increase further.
The similarities to portable soup should be self evident, even if this pottage is grain based, and thus more resembling Rumford's Soup.
However, I do question the depiction of this staple as presented in the series. For "nutrition while working in the fields" this doesn't make much sense to consume it cold and with beer. As you would still have to carry all the ingredients and all the bulk of the dry material and the beer with you. In two containers instead of just one for the entire pottage. So why not carry the fully made pottage with you? There is zero benefit to observe?
The 'instant'-like nature of the pre-made almost powder-like base is good for storage and preparedness — it is much quicker to arrive at a finished pottage than to start with boiling whole grain for hours until it bursts and 'dissolves' — not for convenience 'in the field'?
In any case, a medieval beer thickened with starchy grain powder and toasted ground beans may taste 'OK' and it does provide you with plenty of energy and is approaching a complete protein mixture.
While the above is a reasonable approximation and interpolation, the series was advised by Faye Getz and Andrew Prescott. Both scholars seem to have not researched much into medieval peasant food? Nevertheless, the accompanying book to the series gets explicit on the actual ingredients meant, slightly differing from the on-screen presentation, and sadly without references:
Peasant bread was much healthier than our white, steam-baked, sliced bread: it was brown, like a good wholemeal loaf. Peas and beans were sometimes added, which made it even more nutritious. In the fields people ate a kind of medieval pot noodle, a paste of dried vegetables, beans and bread to which they added ale to turn it into an instant meal.
— Terry Jones & Alan Ereira: "Terry Jones' Medieval Lives", BBC Books: London, 2004. p31–32 (General bibliography pertaining to the theme of that episode on p248). (archive.org)
* Of course, that might not mean that much, as many of the relevant books to consider are simply not available to me. Of special interest seems to be stuff like — Constance Bartlett Hieatt: "An ordinance of pottage: an edition of the fifteenth century culinary recipes in Yale university's ms beinecke 163", Prospect Books: London, 1988.
In that researched manuscript collection we do find examples of:
Here bygynnyth the chapters of divers makyng and dygtyng of potagis and flesch... Canabens/ Canabens with bacon/...