Preheat the oven to 350°F (175°C).
In a skillet, brown the ground beef or turkey over medium heat.
Drain excess fat, then mix in the taco seasoning and a splash of water.
Stir in black beans, corn, diced tomatoes, and chopped onion, mixing well.
Arrange taco shells in a baking dish and fill each with the meat mixture.
Top each filled taco with a generous amount of shredded cheese.
Bake in the preheated oven for 15-20 minutes until cheese is bubbly and golden.
Remove from the oven and let cool for a few minutes before serving.
Garnish with diced tomatoes, cilantro, and serve with sour cream or guacamole.
