In a large heavy skillet cook the onion and the garlic in the oil over moderately low heat, stirring, until the onion is softened, add the beans, and mash about half of them coarse with a potato masher.
Add the tomato sauce, the cumin, the chilies, chili powder, and salt and black pepper to taste, simmer the mixture, stirring, for 5-7 minutes, or until it is slightly thickened.
Working with 1 warmed tortilla at a time (keep the others covered while working with each) spread about 3 tablespoons of the filling down the center of each tortilla.
Working from the sides fold in the ends and then roll the tortillas enclosing the filling.
Arrange the burritos, seam sides down, in one layer in a baking dish, sprinkle them with the Monterey Jack, and bake them, covered with foil, in the middle of a preheated 350°F oven for 10-12 minutes.
Sprinkle with green onions and serve with sour cream and salsa.