In a small skillet, saute the onions with the butter over medium-low heat until the onions are nice and golden brown.
In a small saucepan, combine the green enchilada sauce with the green chilies and heat until very warm.
In the microwave or in the oven (on a baking sheet), melt some cheese all over the top of each tortilla so that it covers most of the surface area.
To serve, stack three cheesy tortillas on top of one another. Spoon green sauce over the top and add store-bought salsa, sour cream and cilantro as needed. Delish and so, so easy!
Save the extra sauce for use on nachos, quesadillas or chicken.