1. Preheat oven to 400°F. Warm oil in a skillet over medium-high heat. Sauté garlic for 1 minute. Add beef; cook, stirring and breaking it up, until no longer pink, about 4 minutes. Add onion; sauté until softened, about 5 minutes.
2. Stir chili powder, cumin, cayenne, if desired, and salt into beef mixture; sauté for 1 minute. Add flour; cook, stirring, for 1 minute. Stir in tomatoes with liquid and 1/4 cup water; stir in beans with liquid. Cook, stirring, until thickened, about 2 minutes. Transfer mixture to an 8-inch baking dish.
3. Prepare muffin mix as box label directs. Fold in scallions. Spread muffin batter over beef mixture. Bake until top is golden, 20 to 25 minutes.