Separate the clusters of oyster mushrooms into individual mushrooms. Trim off and discard the tough lower stems. Cut the mushrooms into thin, lengthwise strips. Pour 1 Tbsp. of the oil into a large skillet set over medium-high heat. Add the mushrooms, garlic, salt and pepper and cook until the mushrooms are tender, about 3 minutes. Remove from the heat.
Set a piece of rouladen flat on a work surface. Place 1/8 of the mushrooms in a row at one end of the meat. Roll the meat into a cylinder and secure with a toothpick. Repeat with the remaining rouladen and mushrooms.
Preheat the oven to 350 degrees F. Pour the remaining oil into a large skillet set over medium-high heat. Season the rouladen with salt and pepper. Brown the rouladen well and then transfer to a 9" x 13" casserole. Remove any excess oil from skillet, and then add the stock/flour mixture and wine. Bring to a boil, and then whisk in the mustard and tarragon. Pour this mixture over the rouladen. Cover and bake 60 to 70 minutes, or until meat is tender.