Preheat oven to 350 degrees F.
In a medium skillet, heat the olive oil over medium heat. Add the garlic, onion, and cumin and cook until the vegetables are slightly softened, about 2 minutes. Add the drained spinach and half of the salsa and cook until warmed through. Remove from the heat and stir in the chicken. Season with salt and pepper, to taste.
Spread 1/2 of the remaining salsa in the bottom of a 10-inch square baking dish or oven-proof skillet. Lay 1 tortilla on the salsa and spread 1/2 the filling over it. Top with another tortilla and spread the remaining filling over it, and top with the last tortilla. Spread the remaining salsa over the tortilla and scatter the cheese over the top.
Bake for 20 minutes, or until heated through and cheese has melted. Remove from heat and let stand for 5 minutes. Top with green onions, if desired, and cilantro. Top each serving with no more than 1 tablespoon light sour cream, if desired, and serve.