Shred the raw brussels sprouts. You can do this by hand, cutting in half first so it doesn't roll, then finely slice. Alternatively you can shred them in a food processor or buy pre-shredded.
Finely slice the red cabbage, or use a mandoline or food processor. Cut out and discard the hard white core.
Peel and grate the carrots. Slice the spring onions.
Add the mayonnaise, yogurt, lemon zest and juice, apple cider vinegar, mustard, sugar, salt and pepper to a large bowl and whisk to combine.
Add the shredded brussels sprouts, cabbage, carrots and spring onions to the bowl and stir well. Refrigerate for at least 30 minutes for the flavors to infuse.