Preheat the oven to 350°F.
Pat the lamb dry and season with the salt and pepper.
Heat the olive oil in a large Dutch oven over medium-high heat. Working in batches, brown the lamb on all sides until nicely golden. Transfer the lamb to a plate and set aside.
Add the onions, carrots, leeks, and garlic to the pot. Season with a generous pinch of salt and pepper and cook for about 5 minutes, stirring occasionally, until the vegetables begin to soften.
Return the lamb to the pot. Add the thyme, bay leaf, and broth, and bring to a gentle simmer.
Arrange the potatoes evenly over the top of the stew. It’s fine if they are not fully submerged. Season the potatoes lightly with salt and pepper.
Cover the pot and transfer it to the oven. Bake for 1 hour, until the lamb and vegetables are tender.
Turn off the oven, remove the pot, and gently stir to combine the potatoes with the broth and vegetables, being careful not to crush them. Return the uncovered pot to the warm oven and let the stew rest for 10 to 15 minutes before serving.
Ladle into bowls and serve hot.