Heat oil in large skillet over medium-high heat. Add lamb and onions. Cook until lamb is brown and onions are soft, about 5 minutes. Transfer mixture to a slow cooker.

Add carrots, celery, mushrooms, potatoes, tomato paste, beef broth, beer, Worcestershire sauce, rosemary, salt, and pepper; stir to combine and close slow cooker.

Cook on High for 7 hours. Add peas and cook an additional 15 minutes. Season with salt and pepper.