Preheat your oven to 400°F (200°C). Lightly grease a 9x13 inch baking dish.

In a large skillet or Dutch oven set over medium-high heat, brown the ground beef, breaking it up as you go. Once browned, drain any excess fat. Add the chopped onion and sauté until transparent, about 5 minutes.

Stir in the minced garlic and cook for 1 minute until fragrant. Add the taco seasoning and stir it into the meat mixture. Cook for another minute to bloom the spices.

Pour in the drained diced tomatoes with chilies (Rotel) and the tomato sauce. Bring the mixture to a simmer. Reduce the heat and let it cook, uncovered, for about 8 to 10 minutes until the sauce has reduced and the mixture is thick. Remove from heat. Stir in black beans if using.

Arrange the taco shells upright in the prepared baking dish, lining them up tightly if needed.

Spoon the warm beef filling evenly into each shell, making sure they are substantial but not overfilled. Sprinkle about 1 tablespoon of shredded cheese directly onto the meat filling in each shell.

Sprinkle the remaining cheese liberally along the top edge of each taco.

Bake in the preheated oven for 10 to 12 minutes, until the shells are crispy and the cheese is melted and golden brown.

Remove from the oven and let rest for 2 minutes. Serve hot with toppings like shredded lettuce, diced tomatoes, sour cream, or salsa.