Thaw chicken and bring to room temperature.
Preheat oven to 220C/450F (standard) or 200C/430F (fan/convection). Put the rack in the middle.
In a bowl mix garlic powder, salt, pepper, zest of lemon and thyme. This is the dry seasoning mix.
In another bowl mix melted butter and fresh lemon juice.
Pat dry chicken really well with a kitchen towel (lesser the moisture, more crispy is the skin).
Pout butter and lemon juice all over, inside the cavity. Loosen the skin with the back of a spatula and pour butter mixture underneath the skin as well.
Insert used lemon wedges inside the cavity.
Tie legs with a kitchen twine and tuck wing tips under the chicken. (watch video to see how to truss chicken).
Sprinkle the lemon thyme seasoning generously all over.
Place onion and garlic in the roasting pan, place chicken (breast side up) on the bed of garlic and onion. Pour chicken stock around. Place a couple of thyme sprig in the pan.
Place the roasting pan in the middle rack. Roast for 1 hour 15 minutes. Baste chicken with the pan juice every 30 minutes.
The chicken is done when the juices run clear when chicken thigh is pierced with a skewer.
At the end baste again and broil for 2-3 minutes for extra crispy skin.
Cover and let it rest for about 10-15 minutes before serving.