In a large non-reactive bowl, combine the cabbage, peppers, and onions.
In a small saucepan, combine the vinegar, honey, celery seed, mustard seed, and red pepper flakes, and bring to a boil. Reduce the heat and simmer for 5 minutes. Pour over the vegetables and stir to combine. Season with the salt and black pepper.
Let cool, then cover and refrigerate, stirring occasionally, for at least 12 hours and up to 2 days.
Yield: 6 servings