Preheat the oven to 400 degrees F. Place a skillet over medium heat and add the butter, onions, and garlic. Sauté for 3-5 minutes to soften.
In a large bowl mix the sautéed onions and garlic, chopped chicken, sour cream, salsa verde, chopped green chiles, cilantro, and cumin.
Tear the tortillas into rough pieces. Spoon one-quarter of the chicken filling in the bottom of a 3-quart baking dish. Top with 1 cup shredded cheese. Then spread a single layer of torn tortillas over the top. Repeat layering the chicken filling, cheese, and tortillas three layers high, using up all the tortillas. Spread the last one-quarter of chicken filling and 1 cup of cheese over the top.
Bake for 20-25 minutes, until golden and bubbly. Wait at least 5 minutes before cutting. Serve warm!
This chicken casserole does freeze well! Leave it out at room temperature to thaw completely before baking.