Preheat oven to 400°F.
Cut a pocket into each chicken breast and season the inside and the outside of the breast with salt and pepper. Set aside.
In a large skillet over high heat cook 1/2 cup of onions with the butter until translucent, about 5 minutes.
Add the garlic and the mushrooms, cook until soft and browned about 10 minutes.
Remove from heat and pour into a bowl. Stir in the cheese and thyme. Add the breadcrumbs in increments until the mixture is stiff enough to form a loose ball.
Stuff the mixture inside the chicken breasts.
In the same skillet over high heat sear the chicken breast in the olive oil just until brown. Place on a baking sheet and put them in the oven to finish, about 10 minutes.
Meanwhile add the remaining 1/2 cup onions and 1 clove garlic to the pan and cook until soft. De-glaze with wine.
Add the chicken stock and cream and season with salt and pepper.
Place the chicken breasts on a platter and pour the sauce over. Garnish with parsley.