A Lighter Lamb Stew

Name: A Lighter Lamb Stew
Ingredients: 1 24-ounce bag of baby Dutch potatoes; 4 Large carrots, cut into 1-inch pieces; 1 Large yellow onion, chopped; 1 3-pound boneless lamb roast; 1 6-inch piece fresh rosemary; 5 sprigs fresh thyme; 5 sprigs flat-leaved parsley; Zest of 1 lemon; 4 Garlic cloves, chopped; 3 tablespoons olive oil; Kosher salt and ground black pepper; 1 11 or 12-ounce bottle Guinness beer; 2 cups homemade or low sodium chicken stock; 2 cups cremini mushrooms, sliced; 1 1/2 cup large frozen green peas