Chop the coleslaw mix finer. The bagged tri-color slaw comes in pretty big pieces. Run your knife through it on a cutting board until everything is in nice small bites — it's much easier to eat that way.
Pre-treat the cabbage. Pour the chopped mix into a large bowl. Sprinkle the 1 tablespoon of sugar and 1/2 teaspoon of salt over the top and stir well. Set it aside while you make the dressing. This step is the whole secret — it pulls the water out of the cabbage so your slaw doesn't go soupy in the fridge later.
Start the dressing. In a separate bowl, add the softened cream cheese. Use the back of a spoon to smooth it out so there are no lumps. This little bit of cream cheese is what gives the dressing that extra creaminess and tang.
Build the dressing. Add the sour cream, mayonnaise, sugar, salt, pepper, celery seed, apple cider vinegar, and lemon juice. Whisk it all together until smooth. Give it a quick taste — it should be creamy with a nice tang. (Don't worry that it tastes strong on its own. It mellows once it hits the cabbage.)
Dry the cabbage. Come back to your bowl of cabbage and give it a stir. You'll see it's glistening — that's all the water the salt and sugar pulled out. Grab a few paper towels and squeeze handfuls of the cabbage gently to wring out the moisture. Work your way around the bowl. You'll be surprised how much water comes out.
Add the onion. Stir the diced sweet or yellow onion into the cabbage. A note here: don't use white onion. It's too sharp and it overpowers everything. Sweet or yellow gives you the flavor without the bite.
Dress and chill. Pour the dressing over the cabbage and onion and stir until everything is coated. Cover the bowl and chill for at least 2 hours before serving. Overnight is even better — those flavors really come together.
Taste and serve. Give it one final stir and a taste before serving. Adjust salt if you'd like. Serve it as a side for burgers, hot dogs, BBQ, fried chicken, or piled on top of a slaw dog. It's also wonderful next to a slice of cornbread.