American Lamb Shepard's Pie with Mushrooms, Stout, & Horseradish Potatoes

Ingredients

• 2 lbs American lamb stew meat or shoulder or leg cut into 1in cubes
• Salt
• pepper
• 2 tbsp Olive oil
• 6 tbsp Unsalted butter
• 1 large Yellow onion, diced
• 3 Carrots, peeled & sliced into 1/4in thick rounds
• 3 stalks Celery, sliced 1/4in thick
• 8 ounces Cremini or button mushrooms, halved
• 2 cloves Garlic, minced
• 6 tbsp All-purpose flour
• 2 cups Lamb stock, such as Saffron Road, or low sodium beef broth
• 1 cup Irish Stout, such as Guinness
• 2 tsp Fresh rosemary, minced
• 2 1/2 lbs Yukon Gold potatoes, peeled & cut into large chunks
• 1 cup Sour cream
• 3 tbsp Prepared horseradish
• 1 cup Frozen peas