Put each chicken breast half between two sheets of plastic wrap; pound lightly to flatten evenly.
Salt and pepper both sides of the chicken pieces.
Lightly oil the grill or grill pan. Grill over a medium-high heat for about 3 to 4 minutes on each side, or until cooked through and set-aside.
In a frying pan, heat the butter and fry the onion until soft and transparent.
Add the wine and boil rapidly until reduced to a quarter of the original amount.
Stir in the cream and mustard and continue to cook until thickened.
Season with the salt and pepper.
Place the chicken on a plate and spoon the sauce over the chicken.
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Key Ingredients: Chicken, olive oil, onion, butter, white wine, mustard seeds, cream, black pepper, salt