Divide tortillas into 2 batches and wrap in aluminum foil. Warm in oven for 15 to 20 minutes. Remove from the oven and lay the tortillas flat on a work surface. Spoon beans, cheddar, onions and peppers down the middle of each, dividing evenly and leaving a margin of about 1 1/2 inches from the bottom edge.
Fold the bottom of the tortilla up, then roll it from one side into a cylinder. Place the burritos on a baking sheet and bake until the cheese is melted, about 5 minutes. Serve at once. If desired, garnish with sour cream and scallions (or chives).