Survey says the information about the pre-fight provisions of fish, eggs, sorrel, and mustard is preserved in the Bruges city archives, which means I can't go look whether the original language holds more info. (I was hoping it was from one of the chronicle accounts of the battle, which I might be able to track down more easily).

Nevertheless, the mention of sorrel and mustard is key in understanding the gist of the food. The herb sorrel, in the Middle Ages, was sometimes cooked into a sauce. Perhaps more often, it would be pressed into verjuice, an acid for cooking (think lemon juice or vinegar). Mustard is definitely a reference to the spice.

Le Viandier, a famous cookbook/recipe collection probably compiled around 1300, suggests that these four ingredients often traveled together. Mustard and verjuice (from crushed sorrel) makes a fine sauce for fried fish; after you've fried up some eggs, use the leftover oil to flavor a mustard/wine "soup" as sauce.

That doesn't necessarily mean the fish was freshly cooked--sauces could easily be used to make salt fish more palatable, and cookbooks at this time were often more prestige descriptions rather than handbooks for in-the-field preparation. Given the quantities and the setting, it's probably more likely that provisions were drawn (or acquired by...persuasive force...) from extant supplies of fish salted for preservation/transport and sale. Eggs were the ubiquitous protein of later medieval Europe, evidently consumed by peasants as well as nobles (we're still in pre-Great Famine days here, with a lot of paying taxes in kind, and eggs were frequently a key component of peasants' taxes). However, the particular combination of ingredients leads to certain general ideas about the elements of the dish and some ways they might have been combined.