Preheat oven to 375. Line a 9x13 pan with aluminum foil. In a food processor place mushrooms. Process until finely chopped. Add garlic, 2 t. thyme and black pepper. While processor is running slowly add olive oil until mixture is combined and begins to cling together. Rub mushroom-garlic mixture on oustide of roast and set aside.
In large bowl combine flour, salt, and 3 T. thyme. Stir in 1 c. of water to form a dough. If dough is too dry add 1 T. of water at a time. On a lightly floured surgace roll dought to twice the size of the roast. Place roast on one side of the dough. Fold remaining dough over roast. Wet seams with a little water and pinch to seal (make sure thare are no holes).
Roast for 1-1/4 hrs. (for rare) or 2 hrs. (for medium). Remove from oven and let sit for 15 minutes.Remove salt crust by tapping with a meat mallet. Pull crust away and discard. Slice thinly. Serve warm or cold.