Preheat oven to 400°. Make slits in each chicken breast, being careful not to cut through completely. 

Cut the zucchini and tomatoes in half length wise, and thinly slice into half-moon shapes. Thinly slice the bell peppers and red onion.

Stuff the cut vegetables into the open slices of the chicken breast.

Spray the stuffed chicken with the spray oil and season with Italian seasoning, salt, and pepper. Sprinkle with the mozzarella cheese.

Bake until chicken is cooked through and no longer pink inside, about 25 minutes.

I topped my chicken with a couple of tablespoons of pasta sauce and a sprinkle of parmesan cheese just before I took it out of the oven!