Preheat the oven to 415°F (210°C).
In a large skillet over medium-high heat, cook the ground beef with the taco seasoning, breaking it up, until no pink remains.
Stir in the chopped olives, diced green chilies, and drained salsa; season with salt and pepper to taste. Remove the skillet from the heat and let the mixture cool to room temperature.
Once cooled, stir 3 cups of the Mexican cheese into the beef mixture until evenly combined.
Spoon about two heaping tablespoons of the beef-and-cheese mixture into each pasta shell (or other shell if using). Place the filled shells in a 9x13-inch baking dish in a single layer.
Top the filled shells with the fresh mozzarella and the remaining 1 cup of Mexican cheese, spreading evenly.
Bake for 15–20 minutes at 415°F, or until the cheese is melted and bubbly.
Remove from the oven and transfer the baking dish to a wire rack; let cool 5–7 minutes.
Top with cilantro, diced tomatoes, diced avocado, and sliced serrano peppers or jalapeños as desired, then serve immediately.
