Melt the butter in a large pot over medium heat, then add the onion, carrot, and parsnip. Cook until the onion is translucent, about 5 minutes, stirring occasionally until you see soft, glossy edges.

Add the potatoes, broth, caraway seeds, bay leaves, salt, and pepper, then bring the mixture to a boil. Once boiling, reduce the heat to a gentle simmer and cook until the vegetables are tender, about 20 minutes, with steady small bubbles.

Remove the bay leaves, then stir in the dried marjoram for the most flavor at the end of cooking. Let it sit in the hot soup for 1 minute off the boil so the aroma blooms without overcooking.

Whisk a ladle of hot broth into the sour cream to temper it, then whisk until smooth and pale. This prevents curdling; aim for a silky consistency before adding it back.

Stir the tempered sour cream mixture back into the pot, then heat gently without boiling. Continue until the soup turns pale and creamy and is hot throughout, then garnish with fresh dill before serving.