If you make the pickled onions and cilantro sauce ahead of time, these happy-making vegan/vegetarian tacos are quick to put together. They're equally good with black beans and pinto beans, so I make them with whatever we have in the pantry.
Heat the olive oil in a medium pot over medium-high heat. Add the onion and pepper and cook, stirring occasionally, until beginning to soften, about 5 minutes. Add garlic and cook, stirring, one minute more.
Stir in the beans, tomatoes (if using whole tomatoes, crush in your hands before adding) or broth, chili powder, and cumin, and bring to a boil. Partially cover, reduce heat to maintain a gentle simmer, and cook, stirring occasionally, until juices have thickened and flavors have melded, about 15 minutes. Mixture should be quite thick so it won't escape from taco shells, but not bone-dry. Season with salt and pepper to taste.
By chili powder, I mean the American-style spice blend that contains some fractional percentage of ground hot chili peppers along with other herbs and spices. I learned the hard way from a reader that chili powder means straight-up ground chili peppers in some other English-speaking countries. If you can't find the spice blend, add a generous pinch of ground cayenne or other hot red pepper, plus 1/2 teaspoon each of dried oregano and granulated garlic.