Place the mushrooms, onion, garlic, stock, tomato paste and lemon juice into a pan and bring to a boil. Reduce heat, cover, and let simmer for 15 minutes, or until the onion is tender.
Blend the cornstarch with the water in a small bowl and stir into the mushroom mixture. Return to a boil, stirring constantly, and cook until the sauce thickens. Reduce the heat and let simmer for an additional 2 minutes, stirring occasionally.
Just before serving, remove the pan from the heat and stir in the yogurt, making sure that the stroganoff is not boiling or it may separate and curdle. Stir in 2 tablespoons of parsley and season with pepper. Transfer the stroganoff to a warm serving dish, sprinkle with remaining parsley, and serve at once with brown or white rice and a crisp green salad.