preheat oven to 425

Heat skillet and oil over medium heat. Sprinkle chicken with half of salt and pepper. Cook chicken 4 minutes each side.

Remove chicken and place in 11x 7 glass baking dish. Bake at 425 for about 15 minutes or until 165°F.

Remove chicken from dish and save drippings.

Return skillet to heat add butter and swirl to coat. Add onion and thyme. Saute 5 minutes.

Combine 3 tablespoons stock and flour in bowl and whisk till smooth. Add flour mixture, remaining stock, and drippings to pan. Scrape pan to loosen browned bits. Bring to boil and cook for two minutes until slightly thickened.

Remove from heat, add mustard and remaining salt and pepper, stirring with whisk.