Add 1 large onion to the bottom of the roasting pan along with whole head of garlic, 8 sprigs of thyme, and 2 sprigs of rosemary. Set aside.
In a small bowl, add garlic clove, salt, black pepper, rosemary, thyme, and oregano. Using a spoon, mix. Remove 1 tbsp of the mixture and add it to another small bowl with 1/3 cup unsalted softened butter. Mix, then set aside. To the bowl with the remaining herb mixture, add 2 tbsp of olive oil & mix together to form a paste.