Peel any loose skins off the garlic bulb. Place garlic bulb in center of foil. Add olive oil and rosemary sprig. Close foil tightly, to keep oil from leaking out.
Place foil pouch in a preheated 250 Fahrenheit oven for 2 1/2 to 3 hours.
Remove foil pouch and allow garlic to cool before opening foil.
Use as desired. (Roasted garlic will hold in the refrigerator for up to 1 week.)