Add the coleslaw mix into a large bowl. Set aside.
Add the mayo, milk, sugar, vinegar, lemon juice, and onion to a blender. Blend until smooth.
Pour the dressing over the coleslaw mix and use a pair of tongs to toss everything together. Make sure the cabbage and carrots are well coated.
Cover the bowl with plastic wrap and store in the fridge until ready to serve. (I highly recommend making the coleslaw at least 2 hours ahead of time for the best flavor. You can even make it the night before.)
I usually like to get a pre-shredded bag of coleslaw mix at the store to cut down on prep. If you prefer to make your own mix, shred some green cabbage and purple cabbage and add in some shredded carrots so you have around 1 pound of veggies total.