Preheat oven to 350°F.
In a skillet, brown ground beef and onion until cooked through, 5-6 minutes. Drain excess fat.
Add taco seasoning, water, and black beans. Simmer for 2 minutes until thickened.
In a bowl, mix together diced tomatoes and sour cream.
Spray an 8-inch baking pan with nonstick spray. Place one tortilla in the bottom.
Layer 1/2 the beef mixture, 1/2 the sauce, and 1/3 of the cheese.
Repeat layers ending with cheese on top.
Cover and bake for 30-40 minutes until hot and bubbly.
Remove foil and bake 5 minutes more to lightly brown the cheese.
Let stand 5 minutes before serving. Top with desired toppings.