Preheat oven to 160°C (140°C fan-forced). Season lamb with salt and pepper. Heat oil in a large ovenproof pot over medium-high heat. Cook lamb, turning as needed, for 8-10 mins or until browned. Transfer lamb to a plate.
Add onion, garlic and thyme sprigs to the pot. Cook, stirring frequently, for 3 mins or until onion browns. Whisk in stock, vinegar and tomato paste. Bring to the boil. Return lamb to pot. Cover and bake, turning lamb over halfway through cooking, for 2 hours or until fork-tender.
Transfer lamb to a plate. Strain cooking liquid into a measuring cup. Allow fat to settle on top. Spoon off fat and discard. Return cooking liquid to pot and simmer over medium-high heat for 15-20 mins or until reduced by half.
Meanwhile, in a large saucepan, add potatoes, a pinch of salt and enough water to cover potatoes by 3cm. Bring to a simmer over medium-high heat. Reduce heat to medium. Simmer for 30 mins or until the potatoes are tender. Drain potatoes and cool slightly.
Using a knife, peel potatoes. Mash, then use the back of a spoon to press through a sieve into the saucepan. (Alternatively, use a potato ricer or mouli.) Stir in butter, sour cream and 1 tsp thyme. Season to taste. Fold egg yolks into potatoes. Transfer to a piping bag fitted with a 1cm plain nozzle.
Line a baking tray with baking paper and pipe 8 mounds of potato mixture onto tray. Using a small spoon, make an indentation in each centre. Refrigerate for at least 20 mins or until firm.
Increase oven to 230°C (210°C fan-forced). Bake potatoes for 15-20 mins or until golden and puffed slightly.
Meanwhile, return lamb to pot with sauce. Simmer over medium heat, spooning sauce over lamb to glaze it.
Transfer lamb to a carving board. Thickly slice. Divide lamb and potatoes among 4 plates. Drizzle sauce over lamb and potatoes. Garnish with watercress.