Soak mushrooms in water for 15 minutes. Remove them and squeeze excess water from them. Reserve liquid. Rinse mushrooms to remove any grit and chop finely. Strain liquid through a very fine sieve.

In a saucepan simmer liquid, mushrooms, garlic and thyme till reduced to 1 tablespoon of liquid. Cool mixture.

Mash the cooled mixture with the butter and season with salt and pepper.

Place mixture in a freezer zip lock storage bag making a log inside of bag. Squeezing all air out. Can be frozen up to 1 month. Just take a slice when needed.