In a large, heavy-bottomed pot, heat the olive oil until smoking. Add the chicken bones, wings and scraps and cook over moderately high heat, stirring frequently, until well-browned all over. Transfer the chicken parts to a plate. Add the carrots, onions and celery to the pot and cook until softened and lightly browned. Return the chicken bones to the pot. Add the water, tomato paste, peppercorns and parsley stems, stir to release any browned bits stuck to the bottom of the pot and bring to a boil. Simmer gently over moderately low heat, skimming occasionally, until reduced by half, about 2 hours. Strain the stock, pressing hard on the solids. Let cool, then refrigerate. Skim the fat from the surface before using.