To prepare steak, sprinkle steak with 1/2 teaspoon pepper and 1/4 teaspoon salt. Heat oil in a Dutch oven over medium-high heat. Add steak; cook 5 minutes on each side or until browned. Remove steak from pan; set aside. Add 1 1/2 cups onion, chopped carrot, 2 tablespoons broth, and sugar. Cover, reduce heat to medium, and cook 10 minutes. Stir in rosemary, mushrooms, and garlic. Cover and cook 2 minutes. Add bay leaves, stout, remaining broth, and steak; bring to a boil. Cover, reduce heat, and simmer 1 hour and 30 minutes or until steak is tender.
To prepare potatoes, place potatoes in a large saucepan; cover with water. Bring to a boil; simmer 20 minutes or until potatoes are tender; drain. Return potatoes to pan. Add butter; beat with a mixer at medium speed just until smooth. Stir in remaining ingredients. Keep warm.
Remove steak from pan; keep warm. Discard bay leaves. Increase heat to medium-high; cook 5 minutes or until slightly thickened. Spoon stout mixture over steak. Serve with potatoes.