Making these Oven-Baked Cheesy Tacos is simple. Just follow these steps:

1. Preheat the Oven: Begin by preheating your oven to 400°F (200°C).
2. Cook the Meat: In a large skillet over medium heat, cook the diced onion until translucent. Add the ground beef or meat substitute and cook until browned, breaking it apart with a spatula.
3. Season: Drain excess fat if necessary. Stir in the taco seasoning and water as directed on the packet. Mix well and let it simmer for about 5 minutes until thickened.
4. Add Fillings: In a large mixing bowl, combine the cooked meat, black beans, corn, and salsa. Stir well to incorporate all ingredients.
5. Prepare Taco Shells: In a baking dish, arrange the taco shells. You may need to prop them up against each other for stability.
6. Fill the Shells: Generously spoon the meat mixture into each taco shell, packing them well.
7. Top with Cheese: Sprinkle an even layer of shredded cheese over all the filled taco shells.
8. Bake: Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the cheese is bubbly and the taco shells are crisp.
9. Garnish: Remove from the oven and sprinkle fresh cilantro on top, if desired.
10. Serve Immediately: Let cool slightly and prepare to serve hot.

These steps will lead you to a batch of Oven-Baked Cheesy Tacos that will thrill you and your guests!