Melt butter in a large soup pot over medium heat. Add the diced onion and carrots, sautéing for about 5 minutes until softened.

Add the sliced mushrooms to the pot and cook for another 5-7 minutes until they release their liquid and start to brown slightly.

Pour in the vegetable broth and add the cubed potatoes. Increase heat to bring the mixture to a boil.

Stir in the rinsed buckwheat groats. Reduce heat to low, cover the pot, and simmer for 15-20 minutes until both the potatoes and buckwheat are tender.

Remove the pot from the heat. To prevent curdling, temper the sour cream by whisking a ladle of hot broth into it in a small bowl, then stir the warm cream mixture back into the main pot.

Season generously with salt and pepper, stir in the fresh chopped dill, and serve immediately.