Saute the onion in butter; add the flour mixed with water to form a paste. Add the bouillon cubes and 2-cups of water, more if you like thin soup. Add the salt and pepper to taste; bring to a boil and simmer for at least 1 hour. Brown the toast; place in the bottom of an oven proof bowl. Top with 2-tablespoons grated cheese. Pour the soup over the top and place in the broiler until the cheese browns or soup bubbles.