Sauté onion in olive oil until tender, about 10 minutes.

Add minced garlic and cook for 30 seconds.

Stir in chopped tomatoes, balsamic vinegar, and basil stalks.

Simmer sauce on low heat for 15 minutes.

Prepare chicken by butterflying and pounding to 1-2 cm thick.

Set up a breading station: coat chicken first in seasoned flour, then dip into whisked eggs, and finally coat with seasoned panko.

Fry the breaded chicken in hot oil for about 5 minutes on each side.

Combine fried chicken on a baking sheet, topping each with tomato sauce, mozzarella, and Parmesan, then broil until cheese melts.

Mix fresh pesto with mayonnaise, toast brioche buns, and assemble sandwich with sauce, chicken, basil leaves, and pesto mayo.