1(10-ounce) can tomatoes with green chiles (such as Ro-tel), with its juices
1cupsour cream, plus more for serving
3cupscooked or 2 (15-ounce) cans pinto beans, drained and rinsed
3cupsshredded Colby-Jack cheese
Preheat the oven to 350°F.
In a large cast-iron skillet, melt the butter on medium-low heat. Add the onions and jalapeño and while occasionally stirring, cook until softened and fragrant, about 5 minutes. Stir in the garlic and cook for 30 more seconds.
Add to the skillet the mushrooms, salt, cumin, chili powder, oregano, and cilantro. While stirring, cook until the mushrooms have reduced, about 5 minutes.
Keeping the heat on medium-low, whisk in the flour and cook for 30 seconds until it’s well incorporated. Whisk in the milk, and while stirring, cook until the sauce has thickened, about 3-5 minutes. Stir in the tomatoes, then stir in the sour cream until creamy and well combined. Taste and adjust seasonings then turn off the heat.
Pour the sauce into a mixing bowl, leaving about 1/4 cup of the sauce in the skillet. Spread it evenly along the bottom. (If you’d prefer to bake the casserole in a baking dish, spread 1/4 cup along the bottom of a 9x13 dish instead.)
Taste the pinto beans and add seasonings to taste if needed. Stir the beans into the sauce.
Tearing the tortillas in half, layer half of the tortillas into the prepared skillet or baking dish, overlapping if needed. Evenly spread half the sauce with beans over the tortillas, then evenly layer half the cheese. Repeat this layering one more time, ending with the cheese.
Bake uncovered for 30 minutes or until brown and bubbling. Allow to rest for 10 minutes before serving. Serve with sour cream, if desired.