Prep ingredients: Preheat broiler with a rack in the top position. Finely chop 2 teaspoons garlic. Trim ends from scallions, then thinly slice. Cut tortillas into 1-inch strips. Drain and rinse black beans.
Cook aromatics: Heat 1 tablespoon oil in a medium ovenproof skillet over medium-high. Add all but ¼ cup of the scallions. Cook, stirring, until wilted, about 2 minutes. Add chopped garlic and taco seasoning; cook, stirring, 1 minute more.
Cook filling: Stir in all of the tomato sauce, ¼ cup water, a pinch each of salt and sugar, and a few grinds of pepper. Add beans and tortilla strips; stir well to combine. Season to taste with salt and pepper. Simmer until sauce is slightly thickened and resembles a loose paste, 2–3 minutes. Top with cheese.
Broil & serve: Broil on upper oven rack until sauce is bubbling and cheese is melted, 2–3 minutes (watch closely as broilers vary). Let rest 5 minutes. Serve enchilada casserole with reserved scallions over top. Enjoy!
