Preheat the oven to 375 degrees and lightly grease a 9x9 baking dish.
In a large skillet over medium heat, cook the ground beef with the diced onion until the meat is browned and the onion is soft. Drain any excess fat.
Stir in the garlic, chili powder, and cumin. Cook for one minute until fragrant.
Add the black beans and salsa to the skillet. Stir everything together and let it simmer for two minutes to thicken slightly.
Place one tortilla in the bottom of the baking dish. Spread one third of the beef mixture over it and sprinkle with one third of the cheese. Repeat the layers two more times, ending with a final tortilla and the remaining cheese on top.
Bake for 20 minutes until the cheese is melted and the edges are lightly golden.
Remove from the oven and let it rest for five minutes. Slice into portions and serve with sour cream and sliced green onions.