Place 6 tortillas into the prepared baking dish. Spread 1/3 of the salsa on top of the tortillas. Spread 1/2 of the meat mixture evenly over the salsa. Sprinkle with 1/2 of the green onions. Drop 1/2 of the sour cream randomly over the green onions. Top with 1/2 cup Cheddar and 1/2 cup Monterey Jack cheese. Repeat layers. Top with 6 tortillas, spread with remaining salsa, and sprinkle with remaining cheese.