Trim steak, beat it flat, and cut into narrow short strips

Saute onion and garlic in 2 tablespoons of butter until soft and starting to brown

Add mush- rooms and simmer on very low heat while turning mushrooms

Transfer the vegetables to a dish and keep warm. Add the rest of the butter to pan, add steak and brown

Reduce heat and add vegetables, dry sher- ry, and mustard

Add yogurt and cornstarch, and heat until sauce becomes hot and thick

Season to taste

Fold in sour cream just before serving and garnish with fresh dill or parsley

Serves 4-6.