Pour off all but 1 tablespoon of fat from the skillet and add the pancetta. Cook over low heat, stirring, until the pancetta is lightly browned, about 4 minutes. Add the shallots, garlic and chanterelles to the skillet and cook, stirring occasionally, until the chanterelles soften, about 5 minutes. Add the potatoes, thyme, chicken stock, wine and the reserved porcinis and porcini liquid and bring to a simmer. Submerge the duck legs in the sauce, cover and bake for 1 hour. Stir in the pearl onions, then cover and bake for 30 minutes longer, or until the duck is very tender.