Cook the buckwheat according to package instructions, typically simmering with water or broth until tender. Set aside.

While buckwheat cooks, heat oil or butter in a large pan or pot over medium heat.

If using, add chopped onion and cook until softened, about 3-5 minutes. Add sliced carrots and cook for another 5-7 minutes until slightly tender.

Add the diced chicken to the pan and cook until browned on all sides and cooked through.

Stir in the frozen mixed vegetables and optional mushrooms (if using) and cook for 5-7 minutes until heated through and slightly softened.

Reduce heat to low, stir in the sour cream, salt, and pepper. Mix well to create a creamy sauce. Do not boil the sauce.

Combine the cooked buckwheat with the creamy chicken and vegetable mixture in the pan. Stir gently to coat everything.

Transfer the mixture to an oven-safe dish. Sprinkle breadcrumbs evenly over the top. If using optional cheese, sprinkle it over the breadcrumbs.

Place the dish under a preheated broiler for 3-5 minutes, or bake at 200°C (400°F) for 10-15 minutes, until the breadcrumbs are golden brown and crispy.

Let stand for a few minutes before serving. Garnish with optional fresh herbs if desired.