A couple of ideas:
blend cashews and water in a blender on high speed. If you have a vitamix, this is all you need to do. With a regular blender, soak cashews in water, and then blend and maybe run through a nut milk bag.
Sautee garlic and shallots in olive oil, add cashew cream, reduce till you have the right consistency, add black pepper cayenne and nutritional yeast.
I've used this for a mushroom stroganoff, which sounds similar to what you are making. No one had any idea it was vegan, and everyone really liked it.
Vegan Sour Cream
(recipe is from 'The homemade vegan pantry' -- which is a nice starting place for a lot of vegan basics... would highly recommend it.)
1 can of coconut milk
2 tbsp non-dairy yogurt (store bought is okay)
Leave in a non-permeable container outside for 24-72 hours, until the taste is right (texture will be off). Refrigerate for 24 hours for it to thicken.