- 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.