Before learning about medieval cooking, I was only familiar with seething from the phrase "seething with rage". Turns out "to seethe" means to cook by submerging in very hot liquid (usually but not always water). Like boiling, but vaguer. (Some of the cookbook quotes do use "boyle", and at least one uses "boyle" and "seethe" interchangably.)

I recommend frose*, which is pretty much scrambled eggs with fish in it. (Or pork, but I haven't had it with pork. I don't remember exactly what kind of fish I used: haddock or similar.) Surprisingly tasty.

*Re-reading it, I see that's actually where they explain seething. Of course it shows up when I stop looking.