Preheat the Oven: Set your oven to 325°F. Arrange the onion wedges in a roasting pan, creating a base for the turkey. Place the turkey breast on top of the onions with the skin facing upward.
Mix the Seasoning: In a small mixing bowl, combine the olive oil, minced garlic, rosemary, lemon juice, kosher salt, and black pepper. Stir until the ingredients are well blended to form a flavorful herb mixture.
Prepare the Turkey: Carefully loosen the turkey skin by sliding your fingers underneath it. Spread half of the prepared herb mixture beneath the skin, ensuring it covers both sides of the turkey breast evenly. Rub the remaining mixture over the surface of the skin. Pour the broth into the pan around the turkey.
Roast the Turkey: Place the pan with the turkey in the oven and roast, uncovered, for about 1 hour 45 minutes to 2 hours. Roast until the skin turns a deep golden-brown color and a meat thermometer inserted into the thickest part of the breast reads 160°F. If the skin darkens too quickly, loosely cover the turkey with aluminum foil to prevent burning.
Rest and Serve: Remove the turkey from the oven and tent it with foil. Let it rest for about 15 minutes, allowing the internal temperature to rise to 165°F. Carve the turkey by slicing the meat away from the bone, then cutting it into even slices. Serve immediately.