Pat the chicken breasts dry, cut into 2-inch cubes, and season with salt and pepper. In a large stainless steel pan, heat 3 tablespoons oil over medium-high heat. Pan-fry the cubed chicken in batches so that it is browned on all sides, then let rest on a plate or bowl after pan-frying.

Lower the heat to medium and add 1 tablespoon of oil and the onions, cooking and stirring occasionally until they are soft (approximately 3 minutes). Add the garlic and cook until soft and fragrant, about 1 minute. Return chicken to the pan!!! Pour in the wine and let cook until it has been almost completely absorbed. Stir in both the tomato sauce and the chicken stock/broth. Cook, partially covered, over medium-low heat for 40 – 45 minutes.

In a large skillet over medium heat, heat the remaining 2 tablespoons olive oil and melt 2 tablespoons butter. Add the mushrooms and cook until the they are browned and cooked through. Remove from heat and reserve.

When the chicken is cooked, remove from the heat and fold in the mushrooms, heavy cream, mustard, and parsley. Taste and adjust the salt and pepper, if necessary.