In a large mixing bowl, combine the sliced cabbage, shredded carrots, and onion. Toss lightly to mix the vegetables evenly and create a crisp base for the slaw.

In a small saucepan, whisk together the vinegar, sugar, celery seed, Dijon mustard, salt, and black pepper. Place the pan over medium heat and bring the mixture to a gentle simmer, whisking occasionally until the sugar has completely dissolved and the flavors meld together.

Gradually pour in the olive oil in a slow, steady stream while whisking constantly to create a smooth, balanced dressing. Bring the mixture to a boil and allow it to cook for one minute, letting it thicken slightly and develop a tangy aroma.

Remove the pan from the heat and immediately pour the hot dressing over the prepared cabbage mixture. Toss thoroughly to ensure every strand of cabbage is coated in the warm dressing. Cover the bowl and refrigerate for at least 2 hours—or preferably overnight—so the flavors can fully combine and the vegetables soften slightly while retaining their crunch.

Serve chilled as a refreshing side dish for barbecues, picnics, or any classic comfort meal.