Melt butter in a skillet and add mushrooms, garlic powder, onion powder, salt, and sauté 6-8 minutes.
Add the room temperature cream and milk and allow it to reduce by a 1/4, stirring often, reincorporating any “skin” that forms on the surface, gently simmering for 8-10 minutes.
Make a slurry with cornstarch and some of the cream-milk mixture. Add the slurry to the pan, and stir to thicken. Once the soup has reduced or thickened to about 2/3 of its original volume, it’s done!
The yield should be 8 oz., equal to one store-bought can of cream of mushroom soup, so if your recipe (for example: green bean casserole) calls for more than one can, scale this recipe up as needed!