Place oven rack in lowest rack position; heat oven to 400°F. Spray one side of each tortilla with cooking spray. With 2 1/2-inch round cutter, cut 5 rounds from each tortilla (if desired, stack 2 tortillas to cut).
On ungreased large cookie sheets, place tortilla rounds with sprayed sides down. Spread each round with beans. Set aside 1 tablespoon of the onions. Sprinkle rounds with remaining onions and cheese.
Bake on lowest oven rack 11 to 13 minutes or until bottoms are crisp and cheese is melted and bubbly. Top each with about 1 teaspoon sour cream and some of the reserved onions.