Using sharp knife, cut marylands and breasts off ducks. Remove as much fat as possible from carcasses; reserve. Discard wings and carcasses.
Combine duck pieces, salt, garlic, bay leaf, thyme and peppercorns in medium bowl; cover, refrigerate until required.
Meanwhile, place reserved fat in large saucepan; cook, uncovered, over low heat, about 1 hour or until fat has melted. Strain mixture through fine sieve into large bowl; discard solids (you will have approximately 2 cups of duck fat).
Preheat oven to slow (150°C/120°C fan-forced).
Rinse duck pieces under cold water; pat dry with absorbent paper. Place duck pieces, in single layer, in large baking dish. Reserve 2 tablespoons of the fat; pour remaining fat over duck. Top up with olive oil, making sure duck is completely submerged. Cook, uncovered, in oven 2 hours.
Boil, steam or microwave potato until tender; drain. Heat reserved fat in large frying pan; cook potato, in batches, until browned. Cover to keep warm.
Meanwhile, make pear and watercress salad. Place mustard, vinegar, sugar and oil in screw-top jar; shake well. Place watercress, pear and dressing in large bowl; toss gently to combine.
Place duck in same large frying pan; cook, skin-side down, until skin is crisp.
Serve duck with potato and salad.
Duck carcasses and wings can be used to make stock. Cooked duck can be stored up to 1 month, completely covered in fat, in tightly sealed glass container, in the refrigerator. To reheat duck, remove from fat, wrap in foil and place in very slow oven (120°C/100°C fan-forced) for about 30 minutes.Note