Mix all but the 2 reserved onions with the sour cream and 2 cups of the Mexican cheese.
Pour 1/3 of the enchilada sauce into the bottom of a 9x12 pan.
Fill each tortilla with an even amount of the cheese mixture, spooning it down the center line of the tortilla. Fold the two unfilled sides to the center, overlapping them, and place the enchilada folded side down in the baking pan. (If the tortillas are too stiff to fold, you can heat them in the microwave under a damp paper towel or carefully steam them for a few seconds over boiling water.)
Pour the remaining sauce over enchiladas in pan. Bake for 30 minutes at 350°F.
Top with cheese and reserved green onions and bake until cheese is melted over the top (about 10 more minutes).
If available, try a more authentic cheese by using Oaxaca cheese (also called quesilla), a white cheese that melts easily with a creamy texture that is often used as filling in Mexican dishes. Queso quesadilla, labeled quesadilla cheese in English, is another creamy white cheese that melts easily.