Saute onions in 2 Tbsp stock, salt lightly. A few minutes later, add mushrooms, 1 tsp dill, 1/2 cup stock or water, soy sauce, and paprika.
Cover and simmer 15 minutes. Melt butter in large saucepan. Whisk in flour and cook, whisking, a few minutes. Add milk and cook, stirring frequently, over low heat about 10 minutes - until thick.
Stir in mushroom mixture and remaining stock. Cover and simmer 10-15 minutes.
Just before serving, add salt, pepper, lemon juice, sour cream, and, if desired, extra dill (1 tsp). Serve garnished with parsley.