Preheat the oven to 350°F. Heat the oil in a high-sided skillet. Sauté the onions until soft and just slightly golden, 5 minutes. Add the garlic, bell pepper, and mushrooms, and cook until the vegetables are tender, about 5 minutes. Stir in the black beans with a little of their liquid and heat through.
Heat the tortillas in a paper bag in the microwave about 1 minute. Lay the warm tortillas on the counter and divide the filling among them and scatter cilantro on top. Roll, turning in the sides, into a neat package. Lay in a baking dish covered lightly with aluminum foil and warm through in the oven, 10 minutes (20 to 30 minutes if they have been made earlier and chilled).
Values per serving: