Trim off the stem ends of the brussels sprouts and remove any wilted outer leaves. Cook the sprouts in 6 cups of boiling, salted water just until crisp-tender. Drain, then submerge the sprouts in ice water to cool. Drain, then transfer to a large bowl. Cut the tomatoes into thin strips and add them to the brussels sprouts. Toss and set aside. In a glass bowl, whisk together the vinegar, oil, garlic, sugar, hot-pepper sauce, and salt and pepper. Pour this sauce over the sprouts and tomatoes. Add the scallions and parsley. Stir, cover, and refrigerate for at least 2 hours.
Note: To boil brussels sprouts, use 1 cup of water for every cup of sprouts.