Place the potatoes in a medium pot; cover with water and bring to a boil over medium-high. Season with salt and cook until tender, about 15 minutes.
Season the lamb with salt and pepper. Heat a cast-iron skillet over medium-high. Add the oil, two turns of the pan. Add the lamb and cook, turning once, until browned, 7 to 8 minutes. Reduce the heat to medium and add the butter and crushed garlic. Using a spoon, baste the lamb with the pan juices, about 2 minutes. Add the wine and simmer until reduced by half, 3 to 4 minutes. Add a splash of stock and let it bubble up. Add the lemon juice and remove the skillet from the heat.
Drain the potatoes and return to the hot pot. Add a splash of stock, the mascarpone, 1/4 cup milk, the herbs and the grated garlic; season. Add half the peas and coarsely mash the potato mixture; season. Stir in the remaining peas and the remaining 1/4 cup milk, if too thick.
Divide the lamb among plates; drizzle with the pan juices. Serve the mashed potatoes alongside.