Heat a grill pan over high heat.
In a small bowl, mix garlic powder, dried basil, oregano, salt, and black pepper.
Pat the chicken dry and season both sides evenly.
Brush the grill pan with olive oil and grill chicken for about 3 minutes per side until cooked through.
Preheat the oven to 375°F (190°C).
Heat a frying pan over medium-high heat. Add cherry tomatoes, shallot, minced garlic, red pepper flakes, black pepper, sea salt, and olive oil. Sauté for 5 minutes until tomatoes blister.
Spoon pesto over each chicken piece, spread gently, and top with mozzarella. Reduce heat to low, cover, and cook until cheese melts.
Mix melted butter with minced garlic. Brush onto bread halves and toast in the oven for 3–5 minutes.
Spread remaining pesto on bread. Add tomato relish, then chicken, and finish with fresh basil. Assemble and serve your Italian Chicken Pesto Sandwich warm.