Toss cabbage, apple, and onions together in a large bowl.
Whisk vinegar, sugar, olive oil, vegetable oil, Dijon mustard, and red pepper flakes together in a saucepan over medium heat; bring dressing to a simmer. Pour hot dressing over cabbage mixture and toss to coat. Cover the bowl with plastic wrap and refrigerate until flavors blend and slaw is cold, at least 1 hour.
If you are making this to last more than a day it is best to salt the cabbage, let it wilt for an hour, then rinse and dry chopped cabbage before the slaw. This removes water from the cabbage, helping it to stay crisp longer.
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