Shred the cabbage into thin 1/8-inch strips and finely dice the onion into 1/4-inch pieces. Place both in a large mixing bowl and set aside. Having your vegetables prepped and ready ensures the hot dressing can be poured over them immediately after cooking, which helps them soften slightly while maintaining their crisp texture.

In a medium saucepan, combine the sugar, vinegar, oil, celery seed, salt, mustard, and pepper. I like to whisk these together before heating to ensure the mustard and spices are evenly distributed throughout the dressing. Bring the mixture to a boil over medium-high heat, then reduce to a simmer and cook for exactly 3 minutes. This short boil helps meld the flavors together and slightly thickens the dressing. Remove from heat and let cool for about 5 minutes—this prevents wilting the cabbage while still allowing it to absorb the warm, flavorful dressing.

Pour the slightly cooled vinegar dressing from Step 2 over the cabbage and onion mixture from Step 1, and stir thoroughly to coat everything evenly. I always make sure to scrape in all the flavorful oil and spices from the bottom of the pan. The coleslaw is best served warm or at room temperature, though you can also chill it for a cold version. It keeps well in the refrigerator for several days and actually tastes better the next day as the flavors continue to meld.