Mash the garlic cloves with the butter, lemon juice and sea salt.
Place chicken in a roasting pan. Season chicken inside and out with salt and pepper. Take half the roasted garlic butter and with your hands spread in under the skin of the chicken.
Place onion, lemon and thyme inside cavity of chicken; using kitchen twine, tie legs together to enclose.
Transfer roasting pan to oven and roast until chicken is cooked through, about 1 hour.