Preheat oven to 400°F. Season chicken with salt and pepper. Lightly spray a grill or grill pan with oil. Cook chicken until no longer pink, about 2 to 3 minutes per side.
Heat a skillet over medium heat. Add oil and garlic, sauté a 30 seconds, add spinach, salt and pepper. Cook until heated through, 2 to 3 minutes.
Place chicken on a baking sheet, divide spinach evenly between the 6 pieces and place on top. Top each with 1/2 oz mozzarella, roasted peppers and bake until melted, about 3 minutes.