Working with half of the bag at a time, chop the coleslaw mix into 1/8-inch pieces (about the width of the carrots). Transfer to a large bowl. Dice enough of the onion into 1/8-inch pieces to yield 1/4 cup. Transfer to a fine-mesh strainer and rinse with cold water for 1 minute to soften the flavor. Gently wring out the onions in a paper towel and then stir into the coleslaw mix.

In a pint jar or other container with a tight-fitting lid, stir together the sugar and 3 tablespoons water until the sugar dissolves a bit. Add the mayonnaise, vinegar, mustard powder, paprika and 1/2 teaspoon salt, put on the lid and shake vigorously until completely combined. Stir the dressing into the coleslaw, then set aside for 10 minutes to marinate and soften, stirring a few times. Add more salt, if needed. The coleslaw will keep, covered in the refrigerator, for up to 1 day.