Start by cleaning your vegetables thoroughly. Shred the cabbage into thin strips using a knife or grater. Grate the carrots and the onion into a large bowl. Mix them together until the colors look bright and even.

In a separate small bowl, combine the mayonnaise and mustard. Add the apple cider vinegar and sugar to the mix. Whisk everything together until the sugar dissolves completely. The dressing should look smooth and pale yellow.

Add the celery seed, salt, and black pepper to the wet ingredients. Whisk again to distribute the spices evenly. Taste a tiny bit to ensure the balance of sweet and salty is right. Adjust the salt if you prefer more savory notes.

Pour the creamy dressing over the shredded cabbage and carrots. Use a large spatula or spoons to toss the vegetables. Ensure every piece of cabbage is well-coated in the sauce. The mixture will look very creamy at this stage.

Cover the bowl tightly with plastic wrap or a lid. Place the coleslaw in the refrigerator for at least one hour. This time allows the flavors to meld together perfectly. It also softens the cabbage just a tiny bit.

Take the bowl out of the fridge right before serving. The dressing may settle at the bottom of the bowl. Give it one more good stir to bring the moisture back up. Serve it cold for the best experience.