Preheat oven to 375°F; lightly grease a 9x13-inch dish with cooking spray.
Heat oil in a large skillet over medium-high heat; cook onion and garlic 2 minutes, until softened.
Add beef; cook, breaking apart, until browned. Drain excess fat.
Stir in taco seasoning, tomatoes with chiles, and 1/2 cup water; simmer 5 minutes, until thickened.
Mix black beans, corn, and 1 cup salsa in a bowl.
Spread 1/2 cup salsa in prepared dish's bottom; cover with 4 tortillas, overlapping as needed.
Layer half the beef, half the bean mixture, 1 cup cheese, and dollops of 1/3 cup sour cream.
Repeat with 4 tortillas, remaining beef and beans, 1 cup cheese, and 1/3 cup sour cream.
Top with final 4 tortillas, remaining salsa, and remaining cheese.
Cover with foil; bake 30 minutes. Uncover; bake 10 more minutes, until cheese melts and edges are golden.
Stand 10 minutes before cutting; serve with desired toppings.