Preheat oven to 325 degrees. Rub 1 stick of butter all over the turkey breast & push some underneath the skin.
In a baking dish, baking pan, or cast iron skillet - place 1 stick of butter in the center of the pan. Put one of the garlic bulbs (with tops removed but bulbs still intact - cloves exposed & facing up) under beside the butter & top with half of the rosemary sprigs. Place the turkey breast on top of the butter, garlic & rosemary. Sprinkle the turkey breast with garlic salt & pepper to taste. Place the other 2 garlic bulbs (with tops removed, cloves exposed & facing up) beside the turkey breast & top the turkey breast with the remaining rosemary sprigs.
Bake at 325 for 2 to 2.5 hours or until an instant read thermometer inserted into the thickest part reads 160 degrees. Transfer the turkey breast to a cutting board & let it rest for 30 minutes. The temperature will continue to rise as it rests to reach 170 degrees.
Carve the breast, serve with gravy (recipe follows) if desired. Serve the roasted garlic alongside the turkey for folks who really love their garlic (like me!) to eat the caramelized roasted garlic cloves with their turkey.