Here's What You'll Need
chicken breasts, flour, oregano, salt, pepper, mushrooms, onion, marsala wine, sherry vinegar, balsamic vinegar, fontina, parmesan, parsley
Mix flour, oregano, salt, and pepper, whisk to combine. Place mixture in a Ziploc bag, then add chicken breasts. Seal and shake to thoroughly coat on all sides.
Use a sharp knife to cut 2-3" wide pocket into the thick side of each chicken breast, being sure not to cut all the way through the other side.
Heat a skillet, then add butter and olive oil. Add chicken breasts and cook for 7-8 minutes. Flip, then cook for another 3-5 minutes. Transfer to a large plate.
Add more butter or olive oil, then add sliced onions. Cook for 3-5 minutes, stirring often, until onions are starting to soften. Add mushrooms and cook an 5 minutes until browned.
Remove 1 heaping cup of sautéed onions and mushrooms, then transfer to a small mixing bowl. Allow to cool slightly.
To the skillet, add marsala wine, sherry vinegar, and balsamic vinegar, then scrape the bottom of the pan. Bring to a boil, lower heat. Allow sauce to reduce while you stuff the chicken.
To the cooled onion and mushrooms, add grated fontina cheese. Stir to mix well, then divide and stuff evenly between chicken breasts. It's okay if it overflows a bit. Really fill 'em!
Place chicken breasts back in the skillet, spoon some of the sauce on top, then transfer to a preheated oven. Bake for 12-14 minutes.
Sprinkle with grated Parmesan cheese, then sprinkle with fresh chopped parsley. Serve immediately!