Place olive oil, parsley, garlic, salt and pepper in a blender and blend until ingredients are smooth, then set aside.
Using kitchen shears cut both sides of back bone to remove it and discard.
You should be able to open both sides of the chicken and lay it flat with breast side up.
Press down hard on breast with both hands, to open up chicken to create one flat surface. You should hear the bones crack as you do this.
Remove excess fat and gristle and rinse off chicken in cool water, and pat dry with paper towels.
Push your hand under the skin of the chicken, to lift it from them flesh. Try not to pierce the skin or lift it away from the chicken.
Place a few sprigs of thyme in the roasting tin, then place the chicken, breast up, in the roasting tin on top of the thyme.
Drizzle two thirds of the blended ingredients over both sides of the chicken and rub it all over.
Tip the remaining third of the blended ingredients between the flesh and the skin.
Scatter the rest of the fresh thyme on top of the chicken.
Cover roasting tin with foil and seal edges, then place in a preheated oven at 400.
Cook chicken for 50 mins.
Remove chicken from oven and remove foil. Be careful not to scold yourself with the steam.
Return chicken to oven and raise the temperature to 425.
Cook chicken until golden.
Allow chicken to rest for 5 mins before cutting into 4, 6 or 8 pieces.