In a medium bowl, whisk together the mayonnaise, vegetable oil, sugar, vinegar, and salt until smooth and well combined. The sugar should dissolve completely into the creamy base. Stir in the poppy seeds and celery seed, distributing them evenly throughout the dressing. Set aside while you prepare the vegetables—I find making the dressing first allows the flavors to meld slightly while the coleslaw gets its final prep.

Place the coleslaw mix in a large bowl and add the finely diced red onion, tossing to distribute evenly. Pour the poppyseed dressing from Step 1 over the vegetables and stir thoroughly until every bit of cabbage is coated with the creamy dressing. Make sure to scrape the bottom of the bowl as you mix to incorporate all the flavoring. I like to use a rubber spatula and really work through the coleslaw to ensure the dressing coats everything evenly.

Cover the coleslaw and refrigerate for at least 2-3 hours before serving. This allows the vegetables to soften slightly while absorbing the dressing flavors. Give it a good stir before serving to redistribute any dressing that may have settled at the bottom. The coleslaw will keep well in the refrigerator for several days.