Start by preheating your oven to 220°C (Gas 7). As it heats, make the herb butter: In a large bowl, season the butter with salt and pepper, then combine the olive oil, and mix. Stir in the lemon zest and juice, crushed garlic, and chopped parsley until well incorporated.
Remove the giblets from the turkey cavity and generously season the cavity with salt and pepper. Stuff the cavity with the onions, lemon halves, garlic halves, and two bay leaves.
If you narrowed down some on the size of the turkey, you might better to expand to both sides; Use your palms to gently separate the skin around the top of the turkey breast, ensuring the skin has not cut anywhere. Repeat under each of the legs, working from the undersides of the breast toward the leg and under the skin.