In a 3 1/2 quart electric slow cooker, mix together the beer and tomato paste. Whisk in the flour, salt, pepper and garlic until well-blended. Stir in the onion, mushrooms, carrots and potatoes. Add the beef shanks and push down into the vegetable mixture.
Cover and cook on Low heat setting for 8 1/2-9 hours or until the beef and potatoes are tender. Stir in the lemon zest and lemon juice just before serving.