Preheat the oven to 400 F.
In a pan, heat cooking fat and sauté onion and garlic until the onion becomes soft, which should take about 7 minutes. Add mushrooms and cook over medium heat until all the moisture evaporates. Place the cooked onion and mushrooms in a bowl and set aside.
1 onion, 4 garlic cloves, ½ lb mushrooms
Using a sharp knife, cut a cavity in the thickest part of each chicken breast for the stuffing.
4 boneless chicken breasts
Stuff each chicken breast with the mushroom and onion mixture and add chopped hazelnuts. Tie the breasts with butcher's twine.
¼ cup hazelnuts
In the same pan used to cook the stuffing, add more cooking fat and brown the chicken breasts on both sides.
4 tablespoon Cooking fat
Place the browned chicken breasts on a baking sheet and bake in the preheated oven for 15 to 20 minutes or until cooked through.
Remove from the oven and let the chicken rest while preparing the gravy.
In the same pan, heat up the stock with mustard and sage, bring to a simmer, and let it simmer for a few minutes to thicken to the desired consistency.
½ cup chicken stock, ¼ cup Dijon mustard, 2 tablespoon sage, Sea salt and freshly ground black pepper to taste
Cut the twine off the chicken breasts and serve topped with the pan gravy.