1. Mince the mushrooms finely in a food processor or with a very sharp knife. (I used the chopping blade on my Magic Bullet.)
2. Melt butter in a large frying pan and add mushrooms, salt, and pepper. Cook until no liquid remains, and the mushrooms are very fragrant, about seven minutes. Cool completely.
3. Spoon a tablespoon of mushrooms onto the edge of each cheese slice, then roll up and lay seam side down on a tray. Place into the freezer until ready to bread.