Heat the oil in a medium sauté pan over moderate heat. Add the chili flakes and stir quickly until fragrant, about 20 seconds. Add the mushrooms and toss around in the pan until slightly wilted, about 1 minute. Remove from heat.

Combine the soy sauce, salt, lime juice, brown sugar, lemon grass, lime leaves, and onion in a mixing bowl and stir until sugar dissolves. Add the mushrooms, cucumber, mint leaves, and tomatoes. Toss gently to blend the flavors. Set aside.

Meanwhile, halve the cabbage and save half for lining plates. (If cabbage is too big, cut the top third off. Otherwise, the leaves will be too long.) Cut into 6 wedges, about 2-inches thick. Wash and drain.

To serve, line salad plates with the leaves from reserved cabbage half. Top with mushroom mixture and garnish with cilantro. Serve with cabbage wedges. The traditional way of eating this dish is to use the cabbage leaves from the wedge as spoons to scoop up the mushrooms.