In a deep pot, saute garlic and onion in oil for 2 or 3 minutes. Add broth and bring liquid to a boil. As you slice potatoes, add them carefully to the broth. Cook potatoes 20 minutes, stirring occasionally. The potatoes will begin to break up and thicken broth as the soup cooks. Stir in spinach in bunches as it wilts into soup. Season soup with nutmeg, salt and pepper, to your taste. Stir in tomatoes and heat through, 1 or 2 minutes. Remove pot from the stove to a trivet. Stir grated cheese into your soup and serve.