In a 10- to 12-inch nonstick frying pan over high heat, cook tortillas, in a single layer, until soft and lightly toasted, turning once, about 3 minutes per batch. Set aside as toasted.
Reduce heat to medium. Add onion and stir until limp, about 5 minutes; remove from pan.
Mix ground chilies and flour in pan and stir over low heat until chilies smell lightly toasted, about 2 minutes.
Whisk in broth until smooth. Stir over high heat until sauce boils, about 2 minutes. Add salt to taste.
Dip 1 tortilla in chili sauce, coating both sides, then set on a microwave-safe plate. Top tortilla with half the onion and a third of the cheese. Repeat layers, using all tortillas and onion. Pour remaining chili sauce (it thickens as it cools) over this stacked enchilada, then sprinkle with remaining cheese.
Heat enchilada in a microwave oven at full power (100%) until hot in center, 1 1/2 to 2 minutes.
Top with sour cream and accompany with beans, lettuce, and tomato.