Preheat the oven to 350 degrees F. Place 1 large onion, peeled and sliced, on the bottom of a roasting pan. Then place 1 9 lb bone-in turkey breast, skin side up, on a rack in the roasting pan. You can use a lightly greased 9x13 pan if you do not have a roasting pan.
In a small bowl, stir together 4 Tbsp melted butter, 2 tsp minced garlic, 1 tsp paprika, 1 tsp dried thyme leaves, 1 tsp salt, and ½ tsp pepper.
Rub the butter mixture on the outside of the turkey breast using a basting brush.
Bake for 13 to 15 minutes per pound, about 2 hours for a 9 lb turkey breast, until the thickest part of the turkey breast reaches an internal temperature of 160 degrees F.
Remove the oven, loosely cover with foil and allow the turkey breast to rest for at least 5 minutes.