Core and slice the apple (using the referenced “apple corer and divider” if you’re lucky enough to have one).
Peel the lemon and cut the peel into thin slices. Juice the lemon.
Cut or break the cheese up into small pieces.
Place the apples, lemon peel and juice, cinnamon stick, salt and cheese into a large pot with enough water to cover everything.
Bring to a boil.
Cover and let simmer for 35 minutes until the apples are very soft.
Stir when you remember to.
Remove the cinnamon stick and blend the soup into a puree. You can pour the soup into a regular blender in batches, or use a nifty immersion blender.
Make a paste out your thickening agent of choice and some cold water. Add to the soup pot.
Cover and simmer until thickened.
Crumble the zwieback and place a handful into the bottom of each soup plate.
Pour the soup on top and serve very hot.