Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Sprinkle chicken with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add chicken to pan; cook 4 minutes on each side or until lightly browned. Remove chicken from pan; place in an 11 x 7?inch glass or ceramic baking dish. Bake at 425° for 16 minutes or until a thermometer registers 165°. Remove chicken from dish; reserve drippings.
Return skillet to medium-high heat. Add butter; swirl to coat. Add onion and thyme; saute 5 minutes or until tender. Combine 3 tablespoons stock and flour in a small bowl, stirring with a whisk until smooth. Add flour mixture, remaining stock, and reserved drippings to pan, scraping pan to loosen browned bits. Bring to a boil, and cook for 2 minutes or until slightly thickened. Remove from heat, and add mustard, remaining 1/4 teaspoon salt, and 1/4 teaspoon pepper, stirring with a whisk. Serve sauce with chicken.