Preheat oven to 375 degrees F. Line a shallow baking pan with foil. In a blender or food processor place mushrooms. Blend or process until finely chopped (may take 1 to 2 minutes). Add garlic, the 2 tsp. thyme leaves, black pepper. Cover and blend or process to chop garlic. While processor or blender is running slowly add olive oil until mixture is combined and begins to cling together. Rub mushroom-garlic mixture on outside of roast; set aside.
In large bowl combine flour, kosher salt, and 3 Tbsp. thyme. Stir in 1 cup water to form a dough. If dough is too dry add additional water 1 Tbsp. at a time (dough will be firm). On a lightly floured surface roll dough to twice the size of roast. Place roast on one side of dough. Fold remaining dough over roast. Wet seams with a little water and pinch to seal (cut away excess dough), making sure there are no holes.
Roast for 1 to 1-1/4 hours. (135 degrees F for rare; to check temperature, stick meat thermometer through salt crust.) Remove from oven and let stand 15 minutes. Remove salt crust by tapping with a meat mallet or small hammer. Pull crust away and discard. To serve, slice meat thinly. Serve warm or chilled for sandwiches. Store up to 3 days in the refrigerator. Makes 8 to 10 servings.