To make the dressing, whisk mayonnaise, sour cream, vinegar, sugar, honey, mustard, poppy seeds, and 1/2 teaspoon pepper in a bowl. Cover and refrigerate at least 30 minutes or up to 3 days in advance.
Toss the cabbage with 1/4 cup kosher salt and set in a colander over a bowl for at least 30 minutes, tossing occasionally, to draw out some of the moisture from the cabbage. Rinse well and pat dry.
While the cabbage is draining, Core the apples and cut them into 1/8"-thick slices. Then, stack the slices and cut them in to 1/8"-thick matchsticks. Grate the carrots on the large holes of a box grater. Slice the scallions thinly.
In a large bowl, combine the drained cabbage, apples, carrots, cranberries, and almonds. Whisk dressing to recombine and pour over slaw. Toss until evenly coated.
I recommend using a knife to shred the cabbage (or buying a bag of shredded cabbage). I tried shredding my cabbage using a food processor (with the shredding attachment). I thought this would be a great, time-saving idea but it totally just pulverized the cabbage beyond recognition.
1/2 cup shredded carrots may be substituted for grating 2 carrots by hand.
Adapted from Serious Eats. They had a lot of great ideas such as draining the cabbage.