I have done this and had good results. It's important to completely rehydrate them before using them. While the are rehydrating you can be sauteeing the onion and garlic in butter, once they are sauteed, stir in your flour, salt and pepper. Cook for a little to get rid of the flour taste. Drain your mushrooms, save the liquid!!!! Fine chop some, leave others bit bigger. Stir into your roux and cook for about 5 minutes, keep stirring. Slowly stir in your soak liquid, cook until thickened. Just before serving, stir in cream or milk product of choice. Arrgh! Heaven!
Maybe it was because I used the blend of dry mushrooms. I might try with just one kind. I did carefully rehydrate the 'shrooms. After 3/4 hour, I had to cut off bits of the oyster mushrooms and some of the shitake. I didn't use those. I did filter the soaking liquid and used some of it.
I followed the directions in the recipe I found (okay, it was recipe for fresh mushrooms) and simmered half the 'shrooms and onions in stock (and soak water) after sauteeing them. Made the roux in a separate pan and stirred in the milk. I pureed the 'shrooms and onions and added the roux-milk mix and the other half of the 'shrooms (sauteed to golden).
It was more work and not nearly as yummy as the cream of garlic I made recently. DH really liked that. He isn't big on 'shrooms. He only likes them as an ingredient, in small quantities. He really did not like his taste of the 'shroom soup, which was no surprise.
