Prepare the Vegetables: finely chop the leek or white onion; peel and grate carrots; clean and chop shiitake mushrooms into bite-sized pieces.
In a large casserole dish, heat the vegetable oil over medium heat. Add the chopped leek or onion and grated carrots, and sauté them gently until they start to soften. Stir in the marjoram and black pepper, blending them well with the sautéed vegetables.
Add the chopped mushrooms to the casserole dish. Add sour cream and mix well, close with a lid, continue to sauté the mixture for about 10 minutes. The mushroom will begin to soften and will release their aroma.
Meanwhile, peel and dice the potatoes into medium chunks.
Add the potatoes to the vegetables in the casserole dish, stir really well. Then pour in 1 litre of water, ensuring all the ingredients are well submerged. Bring the soup to a boil, then reduce the heat to a simmer.
Let the soup simmer for about 15 minutes until the potatoes become tender. Taste the soup and adjust the seasoning with salt and black pepper.
Remove a couple of potato pieces from the soup, mash them, and return them back to the soup to thicken the broth.
Chop fresh parsley and add it to the soup for more flavor. Close with the lid, turn off the heat and let the soup rest for 10-15 minutes.
Serve warm: ladle the hot soup into bowls, making sure each serving has a good mix of mushrooms, other vegetables and broth.