Cut the lamb shoulder into 1-inch (2.5cm) pieces. Heat the oil in a large, heavy-bottomed saucepan over medium-high heat, and when the oil is hot, add the meat, season lightly with salt and pepper, and brown on all sides. This will take 5 to 7 minutes. Remove the lamb from the pan and add the vegetables. Cook, stirring occasionally, until the onions are translucent, which will take about 6 minutes.

Preheat the oven to 325F (163C).

Return the lamb to the pan and stir. Add the thyme, bay leaves, and the garlic cloves to the meat and vegetables, along with the shoulder bone(s), and enough water to cover. When the water comes to a boil, remove it from the heat, cover, and place the pot in the center of the oven. Bake until the lamb and the vegetables are tender through. Remove it from the oven and when it is cool enough to handle, go through the pieces of lamb with your fingers, shredding them.

Place a strainer over a bowl, and tip the lamb into it, to drain. Keep the juices, which you will transfer to a small saucepan.

Increase the oven temperature to 450F (235C).

Line the metal rounds with parchment paper. Trim the paper so that it is flush with the edges of the round. Set the rounds on the prepared baking sheet. Place ¾ cup of the lamb and vegetable mixture inside each round and press on it firmly. Top each round with half cup of mashed potatoes, pressing it gently into the lamb and smoothing the edges. The potatoes will rise above the edge of the metal rounds, which is fine.

Place the baking sheet with the rounds in the center of the oven. Bake until the potatoes begin to turn golden on top and the meat and potatoes are hot through, which will take about 15 minutes.

Heat any sauces from the lamb over low heat.

Remove the rounds from the oven. Using a metal spatula, slide it under each round and transfer to the center of a warmed dinner plate. Surround the timbales with sauce, evenly dividing it among the plates. Serve immediately.