Preheat the oven to 400°F (200°C) and lightly spray a 9x13-inch baking dish with cooking oil spray.
Heat 2 teaspoons olive oil in a skillet over medium-high heat. Add the diced chicken thighs and sear until golden and cooked through, stirring occasionally.
Add 2 teaspoons minced garlic to the skillet and cook about 1 minute until fragrant.
Stir in 1/3 cup barbecue sauce and salt (about 1/2 teaspoon) and cook until the chicken is well coated and the sauce thickens slightly, then remove from heat.
Arrange the 10 taco shells standing up in the prepared baking dish. Place 1–2 tablespoons shredded mozzarella into the bottom of each shell to help prevent sogginess.
Spoon about 2–3 tablespoons of the barbecue chicken mixture into each shell, dividing the chicken evenly among the shells.
Top each filled shell with sliced green pepper, sliced red onion, sliced mushrooms, and sliced olives as desired, then sprinkle the remaining mozzarella over the tops and finish with a pinch of Italian seasoning.
Bake in the preheated oven for about 15 minutes, or until the cheese is melted and bubbling. Serve immediately with optional condiments.