Pour 1 tablespoon of the oil into a deep skillet or casserole and turn the heat to medium-high. Add the meat and cook, stirring, for a minute; then add the onion and cook, stirring occasionally, until it softens and begins to brown, about 10 minutes. Add the peas and 4 cups water and bring to a boil; turn the heat to medium-low and simmer, uncovered, until the peas are completely tender--10 minutes for cooked or canned black-eyed peas, about 30 minutes for frozen.
Stir in the watercress and cook, stirring occasionally, for just a couple of minutes, or until it wilts. Add more water, if necessary. Taste and adjust the seasoning, stir in the remaining tablespoon oil, and serve.
With Minimal effort: The meat can be prosciutto or ham; you can use bacon or pancetta instead, but cook them a little longer. Instead of onion, you can use garlic, leek, or shallots. Black-eyed peas are the traditional choice, but any legume will work perfectly well. For the watercress, substitute arugula or spinach, or any winter green, like kale, mustard, collards, and turnips, all of which will take a little longer to cook--add them to the soup along with the peas. Liquid condiments, like Tabasco or another hot sauce or vinegar, are good here. Increase the amount of meat to 1/2 pound or so (this is especially good with ham), double the amount of greens, and cook the soup until it's thick; you can call it a stew.
Recipe courtesy of Mark Bittman, author, "The Minimalist Cooks Dinner" (Broadway, 2001)