Melt the butter in a medium pan over medium heat, and add the garlic. Cook until softened and aromatic, about 4 to 5 minutes.
Add the broth and bring to a boil over high heat.
When boiling, season with salt and pepper, then add the diced potatoes, marjoram, and caraway seeds. Reduce heat and simmer, covered for 15 to 20 minutes until the potatoes are tender.
If you are adding an egg, pour it in slowly while mixing the soup to create ribbons of cooked egg.
Meanwhile, heat an oven or toaster oven to 350°F. Add the cubed rye bread to a small sheet pan and either drizzle with olive oil or spray with olive oil spray, and toss with your hands to coat.
Toast for about 10 to 15 minutes, stirring occasionally, until golden brown and crispy. Set aside until soup is done.
Serve the soup topped with croutons and parsley, and if desired stir in some cheese.
