2 pounds American lamb stew meat, or shoulder or leg cut into 1-inch cubes
Salt and freshly ground pepper
2 tablespoons olive oil
6 tablespoons unsalted butter
1 large yellow onion, diced
3 carrots, peeled and sliced into 1/4-inch-thick rounds
3 stalks celery, sliced 1/4-inch thick
8 ounces cremini or button mushrooms, halved
2 garlic cloves, minced
6 tablespoons all-purpose flour
2 cups lamb stock, such as Saffron Road, or low-sodium beef broth
1 cup Irish stout, such as Guinness
2 teaspoon minced fresh rosemary
2 1/2 pounds Yukon Gold potatoes, peeled and cut into large chunks
1 cup sour cream
3 tablespoons prepared horseradish
1 cup frozen peas
Preheat the oven to 325˚F. Season the lamb with 1 1/2 teaspoons of salt and several grinds of pepper. In a Dutch oven, heat the oil over medium-high heat. Working in 2 batches, cook the lamb, turning occasionally, until browned on all sides, about 8 minutes per batch. Transfer to a plate.
Add 2 tablespoons of the butter to the pot and melt over medium heat. Add the onion, carrots, celery, mushrooms, and garlic, cover, and cook, stirring occasionally, until the vegetables just begin to soften, about 5 minutes. Uncover, sprinkle with the flour, and stir it in. Pour in the stock, 1 cup at a time, stirring to combine completely with each addition. Add the stout, rosemary, and 1 teaspoon of salt and bring to a boil over medium-high heat, stirring occasionally. Return the lamb to the Dutch oven, cover, and place in the oven to cook until the lamb is tender, about 1 1/2 hours.
Meanwhile, put the potatoes in a large saucepan and add enough water to cover by about 1 inch. Season generously with salt, cover the pan and bring to a boil over high heat. Uncover, reduce the heat to medium and simmer until the potatoes are tender, 20 to 25 minutes. Drain well, then return the potatoes to the pan. Cube 3 tablespoons of the butter and add it to the potatoes. Start mashing the potatoes, and then gradually begin adding the sour cream. Stir in the horseradish, and season with salt.
When the stew is done, remove it from the oven and increase the temperature to 400˚F. Taste and adjust the seasoning. Stir in the peas and pour the stew into a 3-quart baking dish. Spread the mashed potatoes evenly on top, and use a fork to make decorative lines or peaks. Cube the remaining 1 tablespoon of butter and dot the top. Bake until the potatoes are crusty and browned in spots, 20 to 30 minutes. Remove from the oven and let stand for about 5 minutes, then serve hot.