Preheat oven to 375°F (190°C).
Season the chicken with salt, pepper, and garlic powder.
Heat olive oil in an oven-safe skillet over medium-high heat. Sear chicken breasts on both sides until golden brown (about 3–4 minutes per side). Remove chicken and set aside on a plate.
In the same skillet, add sliced onion and sauté until softened and lightly browned. Pour in balsamic vinegar and honey, stirring to deglaze the pan. Reduce heat to medium and simmer for 2–3 minutes until slightly thickened. Stir in Italian seasoning.
Return chicken to the skillet, nestling into the onions and glaze. Scatter tomatoes and zucchini around the chicken and top each breast with mozzarella.
Transfer skillet to the oven and bake until chicken reaches an internal temperature of 165°F (74°C) and cheese is melted, about 10–15 minutes.
Remove from oven, top with fresh chopped basil, and serve immediately.
