Heat the olive oil or butter in a large pot or deep skillet over medium heat.

Add the chopped onion and carrot and cook until softened, about 5-7 minutes.

Add the diced chicken and cook until browned on all sides.

Stir in the minced garlic and cook for 1 minute until fragrant.

Add the dry buckwheat groats to the pot.

Pour in the stock or water, scraping the bottom of the pot to loosen any browned bits.

Bring to a simmer, then reduce heat, cover, and cook for about 15-20 minutes, or until the buckwheat is tender and most of the liquid is absorbed.

Stir in the frozen mixed vegetables and any optional mushrooms (if using). Cook for another 5-7 minutes until the vegetables are heated through and tender.

Remove from heat and stir in the sour cream. Mix until the sauce is creamy.

Season generously with salt and black pepper to taste.

Serve hot, topped with fresh herbs or grated Parmesan cheese if desired.