Thaw frozen spinach in microwave by heating for 2 to 3 minutes. Ring out as much water as possible.
Heat large skillet with olive oil over medium-high heat.
Add in diced onions and cook for 7 minutes or until the onions soften and the edges begin to caramelize.
Lower heat to medium and add in sliced mushrooms, spinach, salt, garlic powder and cumin powder. Cook for about 8 more minutes, until mixture has cooked down.
Warm corn tortillas in the microwave in 30 seconds until soft and pliable.
In a 2-quart baking dish, begin to assemble the enchiladas. Spoon some of the spinach mixture into a warm tortilla. Top mixture with a sprinkle of shredded cheese and roll up like a taquito and place in baking dish, seam side down. Continue until all tortillas have been filled.
Pour the red enchilada sauce evenly on top of the enchiladas. Top with remaining shredded cheese.
Bake for 20 minutes or until cheese starts to bubble.
Serve with chopped cilantro, onions and cotija cheese.