Heat the oil in a large pan over medium heat. When it shimmers, add the onion and season well with salt. Cook until softened, about 3 minutes. Add the mushrooms and cook, stirring rarely until browned, about 10 minutes. Remove from the heat, add 1/2 cup of the salsa and mix until well combined.
Heat the oven to 375 degrees F and arrange a rack in the middle. Wrap the tortillas in damp paper towels or in a microwave-safe plastic bag and microwave until soft and heated through, about 45 seconds.
Spread half of the remaining salsa in the bottom of a 13 by 9-inch baking dish. To make an enchilada, put 2 tablespoons of the mushroom mixture and 1 tablespoon cheese in the middle of a tortilla. Roll up and arrange, seam side down, in the prepared dish. Repeat to make 12 enchiladas.
Cover the enchiladas with the remaining salsa and sprinkle with remaining cheese. Bake until the filling is heated through, cheese is melted and starts to brown, about 35 to 40 minutes. Transfer to a serving platter and serve with sour cream and avocado.