1. Preheat your oven to 375°F (190°C). Lightly grease a large baking dish or rimmed baking sheet with cooking spray or a thin layer of vegetable oil.
2. In a large skillet over medium heat, cook the ground beef or chicken until no longer pink, breaking it apart with a spatula as it cooks—about 6-8 minutes. If using black beans, warm them through with a little seasoning.
3. Drain any excess fat from the skillet, then stir in the taco seasoning mix and 1/2 cup water. Simmer for 3-4 minutes until the mixture thickens slightly.
4. Remove the skillet from heat and stir in diced onions (if using) and half of the shredded cheeses to mix in some melty goodness right away.
5. Arrange the taco shells standing up in the baking dish, careful not to crush them. Spoon the seasoned meat mixture evenly into each shell.
6. Sprinkle the remaining cheeses generously over the filled tacos. Add diced tomatoes on top.
7. Bake in the preheated oven for 10-12 minutes, or until the cheese is melted and starting to bubble and the shells are crisping up at the edges.
8. Remove from the oven and let cool for a couple of minutes. Sprinkle with fresh cilantro and your favorite toppings before serving.

One thing I learned from years of balancing dinner time with two little kids underfoot: don’t skip the cooling step. The first time I served these, I rushed and handed them out hot as lava—let’s just say, melted cheese is delicious but molten cheese lava is a parenting hazard.