Toss the cabbage with the salt in a colander and rest 20 minutes. The shreds look glossy and slightly wilted, with liquid dripping from the bottom. Do not rinse.

Whisk the mayonnaise, sour cream, vinegar, sugar, celery seed, and pepper in the small bowl for 1 minute. The dressing looks smooth and thick with no sugar grains.

Pile the salted cabbage in a flour sack towel, twist it closed, and squeeze over the sink for 30 to 45 seconds. The cabbage should feel like a wrung sponge, with no drips from a squeezed handful.

Add the pressed cabbage, carrots, and green onions to the large bowl and pour in the dressing. Fold with a rubber spatula for 1 minute until every shred is lightly coated with no white streaks.

Cover and refrigerate 20 minutes, or up to 4 hours. The slaw should feel cold, and the dressing should cling to the shreds instead of pooling.

Stir for 10 seconds, taste, and add a pinch of salt or 1/2 teaspoon vinegar if needed. Serve straight from the refrigerator. The slaw looks creamy and bright, with shreds that hold their shape.