Prepare mushrooms: Wipe the mushrooms using a sheet of paper towel to remove any grit or soil (do NOT rinse the mushrooms). You can either slice the mushrooms (no more than 3 mm in thickness) or chop some of them into smaller pieces (as I have done) to give your sauce a slightly different texture.
Cook mushrooms: In a large shallow pan heat the oil and butter until melted and add the mushrooms. Cook over a fairly high heat for about 10 minutes until all the moisture has been absorbed and the mushrooms become lightly browned (this will add more flavour), stirring occasionally. Stir more frequently towards the end of cooking so the mushrooms brown evenly.
Add spices: Stir in the salt, pepper, onion granules and soy sauce.
Add water: Add the hot water and stir.
Add sour cream: In a bowl stir together the sour cream, flour and 1 tablespoon of cold water until smooth. Add 2 tablespoons of the sauce from the pan and stir to combine. Pour this mixture into the pan and stir until thoroughly incorporated.
Cook: Simmer for 1-2 minutes stirring often.
Serve: Remove from the heat, adjust the seasoning if needed, add freshly chopped parsley and serve.
Notes
Do not wash the mushrooms – wiping them with paper towel will suffice.
Cook the mushrooms over a fairly high heat to allow them to brown lightly for more flavour.
Use sour cream with 12-18% fat content.
Stir the sour cream together with the flour and water until very smooth before adding into the mushroom sauce.
Refrigerate leftover mushroom sauce with sour cream, covered, for up to 3 days. Reheat in the microwave.