Heat the oil in a large saucepan over medium heat. Saute the diced onion for 3-4 minutes until it starts to soften.

Reduce heat to medium-low and add the enchilada sauce, black beans, and refried beans to the saucepan. Stir slowly until mixed thoroughly.

Add any “extra goodies” to the sauce (except cheese or avocado) and continue heating and stirring until it starts to simmer. Be sure to stir continuously, else it may start to splatter as it comes to a boil.

Once everything is mixed well, turn off the heat and begin layering the casserole in a lightly greased 9×13 baking pan. Start by adding a thin layer of the sauce to the bottom (if you start with tortillas first, they may burn). Next add a layer of tortillas (see diagram above). Then alternate sauce and tortillas until there’s about 1/4″ of space left (or you run out of sauce or tortillas!), ending with a layer of sauce on top. Be sure all the tortillas are covered by sauce; any tortilla that’s not covered will dry out.

Sprinkle the cheese on top and cover with aluminum foil. "Tent” the foil so it’s not resting on the cheese (this makes it easier to remove later).

Bake at 350°F for 40 minutes. (If doubling the recipe or using a deeper pan, you may want to bake a little longer.) Remove foil and bake for another 10 minutes, until cheese begins to brown. Allow to cool for at least ten minutes then serve with a few slices of avocado on top.