On a heatproof serving platter, layer the tortillas to cover the surface of the platter. Top with 1/3 of the black beans. Place 1/3 of the artichokes on top of the black beans. Sprinkle some of the cheese and tomatoes on top of the artichokes. Repeat the layers 2 more times. Place under the broiler until the cheese melts. Garnish with dollops of sour cream and the chopped scallions.