Sprinkle cutlets with salt and pepper. Warm butter and oil in a large skillet over medium-high heat until butter is foaming. Cook cutlets until browned, turning once, 3 to 4 minutes. Transfer to a plate and cover loosely with foil to keep warm.
Add shallot and bell pepper to pan and cook, stirring frequently, until softened, 3 minutes. Pour in chicken broth and bring to a boil, scraping browned bits off bottom of pan with a wooden spoon. Add corn and mustard and continue to boil, stirring frequently, until sauce thickens a bit, 2 to 3 minutes more. Place cutlets in skillet and turn to coat with sauce. Add chopped parsley and any accumulated juices from turkey plate to skillet; stir. Serve turkey with sauce immediately.