Shepherd's Pie with Mushrooms, Stout Horseradish Potatoes - Catelli Brothers

Ingredients

• 2 pounds American lamb stew meat, or shoulder or leg cut into 1” 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 ¼” thick rounds
• 3 stalks celery, sliced ¼” thick
• 8 ounces cremini or button mushrooms, halved
• 2 garlic cloves, minced
• 6 tablespoons all-purpose flour
• 2 cups lamb stock or low-sodium beef broth
• 1 cup Irish stout, such as Guinness
• 2 teaspoons 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