Preheat the oven to 375F. Cut 5-6 slices evenly along the chicken breast in the chicken breast, leaving about an 1/8th in attached at the bottom.
Season each chicken breast generously with ½ tsp salt and pepper, making sure to season inside the cuts.
Use a sharp knife or mandolin to thinly slice the zucchini, yellow squash, tomatoes, and onion.
Stuff the cuts with the zucchini, yellow squash, tomato, onion, and mozzarella. Set chicken on a baking sheet.
In a small bowl combine oil, vinegar, wild garlic, and remaining ½ tsp salt. Pour mixture all over the chicken.
Place in the oven and bake for 35-45 minutes or until the chicken is no longer pink