Melt the butter in a large sauce pan over medium heat, add the onions and mushrooms and cook until the mushrooms have released their liquids and it has evaporated, about 10-15 minutes. Mix in the flour and paprika and let it cook for 2-3 minutes. Add the stock, soy sauce and milk, bring to a gentle boil, reduce the heat and simmer for 10 minutes. Remove from heat and mix in the sour cream, lemon juice, dill and parsley. Season to taste with salt and pepper