Make your meal deli-style.
In a big bowl, combine the mix with the onion.
In a separate bowl, whisk the salad dressing, veggie oil, sugar, vinegar, salt, and poppy seeds together. Make sure they're thoroughly blended. Pour the dressing mixture over the coleslaw/onion mix and toss it around to coat. Let that stuff chill in the fridge for 2 hours, and you're done.