SHRED collard greens. Stack the leaves and roll them tightly into a cigar shape. Hold tightly. With a sharp knife, shave off each end into ¼-inch shreds. When the thick center vein is reached, rotate the roll and shred the other side. Unroll the remaining veins and tear or chop off any large remaining pieces next to the vein. Discard the thick center vein.
TOSS together collards, carrots and onions in a very large bowl.
STIR mayonnaise, vinegar, honey, mustard, salt and pepper in a measuring cup or small bowl. Mix well.
DRIZZLE over the salad and toss until the collards are well coated. Taste and adjust seasoning if needed.
COVER and refrigerate until serving. Can be made up to a day ahead. Keeps well up to four days.