Heat some olive oil in a skillet (or use lemon juice for low fat), then add the garlic, onion, and mushrooms and saute until the onions are almost, but not quite translucent. Add chili powder and cayenne pepper to taste, and stir for about 30 seconds. Then add the mustard, and continue to stir for another 30 seconds. Add the rinsed, drained black beans, and let the whole mess simmer for a few minutes.
These beans are very good as a part of taco salad, burritos, etc. Theyre also good just folded up in a heated corn tortilla.