|Preheat oven to 375 degrees.
Heat oil in a small skillet over medium heat. Add onion and saute 5 minutes, stirring frequently. Stir in mustard, brown sugar, chili powder, Cumin and pepper. Cook 1 minute more.
Make a pocket under turkey skin of turkey breast with your fingers. Spread onion mixture evenly under skin.
Place turkey on a rack in a roasting pan. Cover with foil and bake for 45 minutes or until juices run clear. Take off foil after 30 minutes to allow the skin to brown. Remove skin before slicing and serve.