In a large skillet, melt the 1 tablespoon unsalted butter over medium heat. Add the 1 pound cremini mushrooms and 1 cup minced onion. Sauté until the mushrooms are tender and the onions are translucent, about 5-7 minutes.
Add the 1 pound lean ground beef to the skillet. Cook until browned, breaking it up with a spatula as it cooks. This should take about 8-10 minutes.
Stir in the 2 teaspoons ground cumin, 1 teaspoon dried oregano, 1 teaspoon salt, and 2 large minced garlic cloves. Mix well and cook for another minute until fragrant. Pour in the 1 cup water and let the mixture simmer for about 5 minutes, allowing the flavors to meld.
Preheat your oven to 350°F (175°C). In a 9x13 inch baking dish, spread a thin layer of 8 ounces salsa verde on the bottom. Take a soft tortilla, place a generous amount of the beef and mushroom filling in the center, and roll it up tightly. Place it seam-side down in the baking dish. Repeat with the remaining tortillas.
Once all the enchiladas are in the dish, pour the remaining 8 ounces salsa verde over the top. Spread 1 cup sour cream evenly across the enchiladas, then sprinkle with 8 ounces Oaxaca cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
