Add the mayo, sour cream, sugar, vinegar, lime juice, and some salt & pepper to a small bowl and whisk together until smooth. Give it a taste and adjust ingredients if needed to suit your tastes (e.g. add more sugar or vinegar).
Prep your apples and scallions.
Add the coleslaw mix to a large bowl, along with the apples and scallions. Pour the dressing over top and toss until combined. Season with extra salt & pepper as needed (this is a fairly big salad so I am generous with both).
Coleslaw can be served immediately or chill it for a few hours first.