Preheat the oven to 400°F (200°C). Spray a 9x13-inch baking dish with cooking oil spray and set aside.
Heat 2 teaspoons olive oil in a skillet over medium-high heat.
Add the diced chicken thighs to the hot skillet and sear until golden and cooked through.
Stir in 2 teaspoons minced garlic and cook about 1 minute until fragrant.
Add 1/3 cup barbecue sauce and salt (about 1/2 teaspoon) to the chicken, stir to coat, and cook until the sauce is glossy and slightly thickened. Turn off the heat.
Arrange the 10 stand-and-stuff taco shells upright in the prepared baking dish.
Sprinkle about 1–2 tablespoons shredded mozzarella into the bottom of each shell to prevent sogginess.
Spoon about 2–3 tablespoons of the barbecue chicken mixture into each shell on top of the cheese, distributing the chicken evenly.
Top the filled shells with sliced green bell pepper, sliced red onion, sliced mushrooms, and sliced olives.
Sprinkle the remaining shredded mozzarella over the shells and finish with a pinch of Italian seasoning.
Bake for about 15 minutes, or until the cheese is melted and bubbly. Serve immediately with your favorite condiments if desired.
