1. Warm 2 tsp. oil in a large nonstick skillet over medium-high heat. Season chicken with salt. Cook 2 breast halves, turning once, until browned and cooked through, 6 to 8 minutes total. Transfer to a plate and cover with foil to keep warm. Repeat with remaining oil and chicken.
2. Add shallot to pan and cook, stirring, until softened, about 30 seconds. Add orange juice and broth and bring to a boil, stirring to scrape up browned bits from bottom of pan. When mixture boils, lower heat and simmer until reduced and thickened, about 5 minutes. Remove from heat and whisk in mustard, parsley and butter. Season with salt. Pour sauce over chicken and serve.