2cups cooked (method above) white beans, such as Great Northern (Dried Bean Basics), or 1can (19ounces) cannellini beans, drained in a sieve and rinsed under cold water
1ripe plum tomato, cored, seeded (Tomato Basics), and cut into ¼-inch dice (optional)
2tablespoonssherry vinegar or wine vinegar, or to taste
Salt, if needed
3tablespoons chopped fresh parsley
1scallion (green portion only), thinly sliced on a sharp diagonal
freshly ground black pepper, to taste
Heat the oil in a nonreactive medium saucepan over medium heat. Add the onion and garlic; cook, stirring with a wooden spoon, until wilted, about 6 minutes. Stir in the diced roasted pepper (with a spoonful of the roasting juices, if you have them), the drained cooked beans, tomato, vinegar, and salt, if needed.
Stir gently over medium heat until very warm but not quite hot. Stir in the parsley, scallion, and black pepper. Taste and correct all of the seasonings, adding more vinegar and salt and pepper if needed, and serve.