Prepare a baking dish by spraying it with cooking spray and heating the oven to 400 degrees.

Butter a big pan and heat it over medium to medium-high heat.

Sliced onions should be added to the melted butter and cooked for 5 minutes while stirring occasionally.

Add the garlic, pepper, and thyme, and lower the heat to medium.

Continue to cook for a minute more.

Add the beef stock and balsamic vinegar and simmer, stirring occasionally, for about 10 minutes, or until the onions have become a jam-like consistency.

Once the onions have caramelized, use a third of the mixture to cover the bottom of a buttered baking dish.

Spread the onions out on a baking sheet and lay your big chicken breast on top of them.

Season chicken breasts very gently with your go-to seasoning.

The last of the onion pieces should be layered above the entire chicken breasts, followed by the mozzarella and parmesan.

Bake the chicken in the preheated oven for 30 minutes, or until an instant-read thermometer registers 165 degrees F.

Enjoy right away for the best chicken dinner with a mouthwatering French onion flavor!