Grilled Chicken in Mustard Sauce

For the grilled chicken
4 boneless chicken breast halves
1 Tbsp extra-virgin olive oil
For the sauce
1 onion, finely chopped
2 Tbsp butter
3 Tbsp French mustard
250 ml thick cream
1/4 tsp salt
1/4 tsp black pepper
Method
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.
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.