Start by slicing your chicken breasts in half lengthwise. This will create thinner pieces that cook more evenly and quickly. Season both sides generously with kosher salt and black pepper.
In a skillet, heat 1 teaspoon of olive oil over medium heat. Add the crushed garlic and sauté for about 30 seconds until fragrant. Then, add the drained spinach and cook for an additional 2-3 minutes, stirring occasionally. Once heated through, remove from heat and stir in the shredded mozzarella and roasted red pepper strips.
Take each piece of seasoned chicken and place a generous amount of the spinach and mozzarella mixture in the center. Fold the chicken over to encase the filling. You can secure them with toothpicks if needed.
Preheat your grill or grill pan over medium-high heat. Lightly spray the grill with olive oil spray to prevent sticking.
Once the grill is hot, place the stuffed chicken on it. Cook for about 6-7 minutes on one side, then flip and grill for another 6-7 minutes or until the chicken is fully cooked and has nice grill marks.
Remove the chicken from the grill and let it rest for a few minutes. This allows the juices to redistribute, making the chicken even more tender. Serve immediately, perhaps with a side salad or roasted vegetables for a complete meal.