In a large skillet, heat the olive oil over medium heat. Add the chopped onion and garlic, cooking until softened (about 3-4 minutes). Add the ground beef and taco seasoning, cooking until the beef is browned, about 5-7 minutes. Stir in the diced tomatoes and black beans, cooking for an additional 2-3 minutes until heated through.
To make the tacos, take each tortilla and spoon a generous amount of the beef mixture onto one half of the tortilla. Fold the tortilla over, pressing gently to secure the filling. Repeat for all tortillas.
Place the folded tacos seam-side down in a greased baking dish. It's okay to keep them snug, as they will hold their shape during baking.
Sprinkle the shredded cheese generously over the top of the arranged tacos.
Place the baking dish in the preheated oven and bake for 15-20 minutes or until the tortillas are slight crispy and the cheese is melted and bubbly.
While the tacos are baking, prepare your toppings. Chop the lettuce and dice the avocado.
Once baked, remove the tacos from the oven and allow them to cool slightly. Serve with shredded lettuce, diced avocado, a dollop of sour cream, and garnish with fresh cilantro if desired.