Put a rack in the middle of the oven and pre-heat to 375° F.
Rinse the chicken and sprinkle the cavity generously with salt and pepper. Insert in the cavity a couple of the garlic cloves, half of the fresh thyme sprigs, and 2 pieces of lemons.
Slice the leek in small rounds and rinse if need be. Place on the bottom of a heavy baking pan. Add the sliced onions and garlic.
Place the chicken on top of the leeks and onions. Drizzle lightly the outside of the chicken with olive oil. Sprinkle generously with salt and pepper. Add the lemon slices all around it and the fresh thyme. Add the chicken stock.
Baste occasionally and poke the chicken legs and breast to let out the juice.