The first step in making French onion chicken Parmesan is to caramelize the onions. To caramelize onions, thinly slice several large, peeled onions, and cook them in a bit of butter until they are deeply browned and soft. Plan for this to take 30 minutes to 1 hour, depending on how many onions you start with.
Pound chicken breasts with the flat side of a meat mallet or rolling pin to an even ½-inch thickness. Season on both sides of the chicken with kosher salt and black pepper, then proceed to breading the chicken.
First, coat the chicken into all-purpose flour and shake off any excess. Next dip the chicken into a dish with beaten eggs, and finally coat in a mixture of Italian breadcrumbs (or panko breadcrumbs mixed with dried Italian seasoning) and grated Parmesan cheese.
Pan-fry the chicken in a large skillet until they are browned on both sides.
Transfer the chicken to a baking sheet and top with caramelized onions, Gruyère cheese, and grated Parmesan cheese.
Bake until the chicken is cooked through, an instant-read thermometer inserted into the center of the chicken should register 165°F, and the cheese is melted.
Garnish this French onion chicken Parmesan with more Parmesan cheese and fresh thyme leaves.
