Preheat oven to 375 degrees. Spray an 8x8 inch glass baking dish with cooking spray.
Heat the olive oil in a large skillet over medium heat. Add the onion and sauté, stirring often, for 4-5 minutes, until they are tender. Add the garlic, cumin, and oregano then season with sea salt and freshly cracked pepper, to taste; cook another minute or two, stirring constantly, then add the drained beans and cook 3-4 minutes, then mash the beans slightly with a potato masher. Add the tomatillo verde sauce and simmer until the sauce is mostly absorbed by the beans, about 5-6 minutes. Turn off the heat and stir in the sour cream, green onions, and fresh cilantro.
While the bean mixture is cooking, open the whole chiles with your fingers and wipe with paper towels to dry them and remove any remaining seeds. In the prepared glass baking dish make a layer with half of the whole green chiles
Next layer half of the bean mixture and half of the cheese. Repeat. Layer slices of tomatoes on top.
Cover with foil and bake 20 minutes. Remove the foil and bake for 20 minutes longer, until the casserole is bubbling. Turn the oven to broil and cook, watching carefully, for a few minutes until the cheese is starting to become golden brown. Remove from the oven and let cool for 5 minutes.
Serve topped with sour cream, green onions, cilantro, and diced avocado and with a side of tortilla chips. Enjoy.