Heat oven. Heat oven to 400°F. Grease a 9 x 13 inch baking dish. 

Sauté onions and garlic. Heat 2 tablespoons of the oil in a large frying pan. Add onions, season with the 1/2 teaspoon salt and 1/4 teaspoon pepper. Let cook, stirring occasionally about 6-7 minutes or until golden brown. Add the 4 cloves sliced garlic, cook a minute more. Scrape onion mixture into a large bowl. 

Brown mushrooms. Heat another 2 tablespoons of the oil in the same pan over medium high heat. Add the 6 ounces sliced mushrooms. Try to get them in as single of a layer as possible. Cook undisturbed until brown on one side, then give a stir and brown the other side, about 5 minutes total. Transfer to the bowl with the onions and let cool.

Combine the filling. Save 1 cup of shredded Oaxaca cheese for the top. Mix remaining cheeses with the mushrooms. Taste and season with salt if needed. 

Heat enchilada sauce. In the same pan you used to sauté the mushrooms, bring the enchilada sauce to a simmer then turn off the heat.

Roll the enchiladas. Heat a comal or dry cast iron skillet over medium heat until hot, but not smoking. Warm one tortilla until soft and pliable on both sides but not toasted, about a minute. Dip warm tortilla in sauce so both sides are coated. Place sauce-covered tortilla on a plate. Add 2 tablespoons of the filling, roll and place seam side down in the baking dish. Repeat with remaining tortillas and filling. If you have extra filling it makes awesome quesadillas or grilled cheese. 

Top with sauce and cheese. Top enchiladas with any remaining sauce that is in the pan then sprinkle with the 1 cup reserved cheese. If the sauce has gotten too thick, thin with some water until it is a pourable consistency.

Bake. Bake enchiladas uncovered for 20 minutes or until cheese is browned and bubbly. 

Serve. Serve with guacamole and Mexican crema.