Preheat the grill to medium-high heat. Season chicken breasts with salt and pepper. Lightly spray the grill with oil. Grill chicken until cooked through, approximately 4-5 minutes per side.

While chicken is grilling, heat a skillet over medium heat. Add olive oil and garlic, sauté for 30 seconds, then add drained spinach. Cook until heated through, about 2-3 minutes, seasoning with salt and pepper to taste.

Once chicken is cooked, transfer to a baking sheet. Spread sautéed spinach evenly over each piece. Top with shredded mozzarella and sliced roasted red peppers. Broil briefly until cheese is melted and bubbly, approximately 2-3 minutes.