Before I made this recipe (again from A Feast of Ice and Fire: The Official Game of Thrones Companion Cookbook), I wasn't sure what pease porridge actually was, having only heard of it in the nursery rhyme. I figured it was like pea soup, which is nearly true. According to wisegeek.org, it originated in Britain when people would make a huge batch of pea soup, let it congeal overnight, then eat it again the next day (and the next, and the next...). So pease porridge, also called pease pudding, is essentially thickened pea soup. And that's what it looks like:
Wikipedia says pease porridge should be "somewhat similar in texture to hummus", and I think I came close! It was really simple to make: you cook some split peas until they're soft, along with the two halves of an onion and a tied-up bundle of herbs (thyme, basil and parsley). Then you beat in an egg to thicken it up, and add salt and pepper. There are two ways to finish it: either boil it in a pudding cloth, or bake it in the oven at 350 degrees for half an hour. I chose the second way, hence the crispy bits on the edges. (Note: I removed the onion and herbs before baking, but you could also blend them along with the peas in a food processor, or chop them by hand and put them back in. I guess I'm just lazy.)
If you like pea soup (or, heck, mushy canned peas) you'll probably like this. It had a subtle flavor from the onion and herbs, but if you wanted to make it bolder you could add pretty much any seasoning you like.
And finally, here's a little linguistic tidbit for you: the word pea comes from the Latin pisum. It was originally borrowed into English as pease, hence the term "pease porridge". Eventually people thought pease must be a plural, so they created the new singular form pea. Wow!