Heat 1 Tbsp. oil in large pan. Add the ham and brown sugar. Sauté over medium heat for 2 minutes. Remove ham and set aside. Add the remaining oil to pan along with the onion, leek and garlic. Sauté until onion is soft and lightly browned. Add flour and continue to cook for 1 minute. Gradually add the broth; then add the potatoes and cook until the potatoes are tender, approximately 15-20 minutes. Add the ham, cabbage and vinegar and simmer for 5-10 minutes while you prepare the dumplings. To make the dumplings, cut butter into the flour with a fork or pastry blender until it is crumbly. Mix in the cheese, thyme and a little water at a time until mixture binds together to make small balls. Place balls into the soup and simmer covered for about 8-10 minutes. Scatter lemon rind over dumplings just before serving. Serve hot.