Heat the oil in a large pan or pot over medium-high heat.

If using, add the chopped onion and cook until softened, about 3-4 minutes.

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

If using, add the minced garlic and cook for another minute until fragrant.

Add the sliced carrots and buckwheat to the pan, stirring to combine with the chicken.

Pour in the broth (or water), season with salt and pepper, and bring to a simmer.

Reduce heat to low, cover, and simmer for about 15-20 minutes, or until the buckwheat is cooked and most of the liquid is absorbed.

Stir in the frozen mixed vegetables and continue to cook, covered, for another 5-7 minutes, or until the vegetables are heated through and tender.

Stir in the sour cream just before serving. Cook for 1 minute more if needed to heat through, but do not boil.

Taste and adjust seasoning if necessary. Serve hot.