Press Sauté in the Instant Pot. Add butter. When butter is starting to melt, add onion. Sauté until soft & tender for 5 minutes.
Then add mushroom and saute for 5 minutes or until all the mushroom liquid is absorbed, and the mushroom turns out a little bit soft.
Add spices and vegetable stock. Close the lid and cancel saute. Pressure cook for 5 minutes. Once done, let the pressure release naturally and quickly release the remaining pressure.
In a bowl, whisk milk and flour until the mixture is smooth and free of any lumps. Open the lid of the instant pot. Press saute and pour the milk & flour mixture, then simmer.
Turn off the instant pot. Add lemon juice, sour cream, and parsley. Mix everything well.