Heat oil in a large sauté pan. Add onions and sauté until golden. Add celery and cook until soft. Add pepper and zucchini, sauté until slightly softened. Season with salt and pepper. Remove from heat and cool 10 minutes.

With a small sharp knife, cut a slit in the side of the chicken and move your knife up and down in an arc to create a pocket in the chicken breast.

Spoon one heaping tablespoon of vegetable mixture into the pocket, stuffing it all the way inside. Repeat with remaining chicken cutlets.

Place beaten eggs in one dish and panko crumbs in another. Carefully dip the chicken into the eggs. Then dip into the crumbs, patting to make sure it is completely covered.

Preheat oven to 375 degrees Fahrenheit.

Place chicken on a foil-lined sheet pan. Drizzle or spray chicken generously with oil. Bake 30 to 35 minutes, turning chicken over halfway through cooking time. Remove from oven and allow to rest 10 minutes. Slice cutlets on the diagonal and serve.