Preheat the oven to 400°F (200°C).
Season each chicken breast (the 6 halved pieces) on both sides with kosher salt and black pepper.
Lightly spray a grill or grill pan with olive oil spray and heat over medium-high. Grill the chicken 2–3 minutes per side, until no longer pink and cooked through; remove to a baking sheet.
Heat a skillet over medium heat and add 1 teaspoon olive oil. Add the crushed garlic and sauté about 30 seconds until fragrant.
Add the drained frozen spinach to the skillet, season with salt and pepper to taste, and cook 2–3 minutes until heated through; remove from heat.
Divide the cooked spinach evenly among the 6 chicken pieces on the baking sheet. Top each with about 1/2 ounce shredded mozzarella and some sliced roasted red pepper strips.
Bake in the preheated oven about 3 minutes, until the cheese is melted. Serve immediately.