Preheat oven to 400 degrees. In a large bowl, combine beef, mushrooms, rice, garlic, ginger, scallions, soy sauce, sesame oil, red pepper, and 1/2 teaspoon salt.
Remove 8 large outer leaves from cabbage (if leaves
are less than 5 inches wide, overlap 2 leaves, side by
side). With a rolling pin, roll each leaf until the stem
end is pliable.
Dividing among leaves, mound meat mixture
toward upturned stem end. Starting from filled end,
holding sides in as you work, tightly roll each leaf
into a bundle.
Arrange cabbage rolls, seam-side down, in a 9-by-13-inch baking dish. Pour 1 cup water over rolls;
cover dish tightly with aluminum foil. Bake until an
instant-read thermometer registers 160 degrees when inserted into a roll, 30 to 35 minutes. Serve drizzled with juices and sprinkled with red pepper, if desired.