Preheat your oven to 350°F and grease a 9x13 inch baking dish with oil or cooking spray. While the oven heats, prepare all your ingredients: finely dice both small onions into 1/4-inch pieces, freshly mince the garlic cloves, rinse and drain the black beans, cut the corn tortillas into 1-inch wide strips, and shred the colby jack cheese. I always shred cheese fresh right before cooking because pre-shredded cheese contains anti-caking agents that prevent it from melting as smoothly and evenly.

Heat 1 tablespoon of olive oil in a large saucepan over medium heat and add one diced onion with 3/4 teaspoon salt. Cook for 5 minutes, stirring occasionally, until the onion softens and becomes translucent—this develops the base flavor. Add the chili powder, cumin, and oregano, stirring constantly for 30 seconds to bloom the spices and release their essential oils. Sprinkle the flour over the mixture and stir for another 30 seconds to create a roux base. Pour in the tomato sauce, chicken stock, and freshly minced garlic, stirring until smooth. Simmer for 5 minutes to allow the flavors to meld together. For a smoother, more refined sauce, you can blend it briefly with an immersion blender, but I prefer to leave it slightly textured for more authentic flavor.

While the sauce simmers, heat a second skillet over medium-high heat and add the ground beef with the diced onion, 3/4 teaspoon salt, and 1/2 teaspoon black pepper. Brown the meat for 5 minutes, breaking it apart with a spoon as it cooks, until no pink remains and the onion is tender. Drain any excess fat from the beef using a colander or by carefully pouring it off. Return the cooked beef to the skillet and add the green chiles and black beans from Step 1, stirring gently to combine. Reserve this filling mixture for layering.

Pour 1/2 cup of the enchilada sauce from Step 2 into the bottom of your prepared baking dish, spreading it evenly to coat the bottom. This prevents sticking and creates a sauce base. Remove the sauce from heat and stir 1/2 cup more sauce into the beef-chile filling from Step 3 to keep it moist and flavorful during baking. Dip 6 of the tortilla strips into the remaining sauce to coat them lightly, then arrange them in a single layer over the sauce in the dish. Spread half of the beef-chile filling over the tortillas, then sprinkle 1 cup of the shredded cheese evenly over the filling.

Dip the remaining 6 tortilla strips in the sauce and arrange them over the cheese from Step 4 in a second layer. Spread the remaining beef-chile filling over these tortillas, then top with the remaining 2 cups of shredded cheese, spreading it evenly across the entire surface. This final cheese layer will create a golden, bubbling top when baked.

Place the assembled casserole in your preheated 350°F oven and bake for 20 minutes, until the cheese is melted and bubbly and the edges are slightly golden. Remove from the oven and let it rest for 5 minutes before serving—this allows the layers to set slightly, making it easier to cut clean portions without everything falling apart.