In a 9x13-inch (23x33cm) casserole dish, spread the onions out in one layer. Pat the turkey thighs dry with a paper towel, and place them on top of the onions.

Combine the melted butter, olive oil, finely minced or grated garlic (you can also use a garlic crusher), salt, and pepper. Rub this marinade all over the turkey thighs, cover, and marinate for at least 8 hours, or preferably overnight.

Take the turkey thighs out of the fridge 1 hour before roasting. Preheat your oven to 400°F/200°C. Roast for 50 minutes, rotating the pan 180° halfway through, or until a thermometer in the thickest part of the thigh registers 165°F. Lightly tent the turkey with foil if it’s browning too fast. Rest for 10 minutes before serving.