In a medium bowl, whisk together mayo, whole milk, vinegar, buttermilk, sugar, salt, black pepper, celery seed, cayenne, and lemon juice until well mixed. If you have time, allow this dressing to sit for at least 15 min, or covered and refrigerated for up to 2 days to allow flavors to blend.
When you're ready to dress the slaw, put cabbage into large bowl. Add mustard and toss to mix thoroughly. Add dressing and toss again, coating evenly. Taste for seasoning and adjust if needed.
