Preheat the oven to 350°F. Lightly coat a 9x13-inch casserole dish with non-stick cooking spray. Set aside.

In a large skillet over MEDIUM heat, cook the ground beef and diced onion until the beef is thoroughly browned and the onion is tender. Add the minced garlic and continue to sauté for another 1-2 minutes, just until the garlic is fragrant.

Sprinkle in the taco seasoning, pour in the water, and stir the mixture well. Let it simmer for a few minutes to allow the flavors to meld.

Stir in the fire-roasted diced tomatoes with their juices, green chilis, and the black beans. Cook until heated through. This is a great time to taste and adjust seasoning if necessary.

Arrange 6 corn tortillas on the bottom of the prepared baking dish, overlapping them slightly to cover the entire surface. Spread one-third of the meat mixture over the tortillas, then sprinkle 2 cups of the shredded cheese on top. Repeat these layers two more times, ending with a generous layer of cheese.

Cover the dish with aluminum foil and bake in the preheated oven for 30-40 minutes. You'll know it's done when the lasagna is bubbly and the cheese has melted.

Once out of the oven, sprinkle the top with the diced tomatoes and sliced green onions for a pop of color and freshness. Let the lasagna sit for about 10 minutes before slicing.

Cut the Mexican Lasagna into squares and serve with a spoonful of sour cream.