Preheat the oven to 350°F. Lightly coat a 9×13-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.