Remove the outer leaves of the cabbage and cut it in half through the core. Cut out the tough core and slice the cabbage into thin strips about 1/8 inch thick, or use a food processor with the slicing attachment.
Transfer the sliced cabbage to a large bowl along with the grated carrot, sliced onion, and bell pepper.
In a small saucepan, combine the apple cider vinegar and sugar. Heat over medium heat, stirring until the sugar completely dissolves, about 2-3 minutes.
Remove from heat and whisk in the oil, mustard, celery seeds, salt, pepper, and optional red pepper flakes.
Pour the warm dressing over the cabbage mixture and toss thoroughly to coat all ingredients evenly.
Cover the bowl and refrigerate for at least 1 hour, though 4 hours will deliver optimal flavor. Toss occasionally to ensure even marination.
Before serving, taste and adjust seasonings as needed. Drain excess liquid or serve with a slotted spoon if you prefer a less wet slaw.