Begin by combining the coleslaw mix and diced onion in a large bowl.
In a separate medium bowl, whisk together the salad dressing, sugar, vegetable oil, vinegar, poppy seeds, and salt until they are fully blended.
Pour the dressing over the coleslaw mixture and toss everything together until the coleslaw is well-coated.
Allow the coleslaw to chill for at least 2 hours before serving. This gives the flavors time to meld together, resulting in a coleslaw that is perfectly tangy, sweet, and oh-so-delicious!