2/3 cup buttermilk
1/3 cup mayonnaise
3 tablespoons fresh lemon juice
Kosher salt, freshly ground pepper
1 small bunch broccoli (about 12 ounces)
1/2 medium Napa cabbage, thinly sliced (about 6 cups)
2 scallions, thinly sliced
8 ounces sugar snap peas, thinly sliced
4 tablespoons chopped fresh chives, divided
Combine buttermilk, mayonnaise, and lemon juice in a small bowl; season with salt and pepper and whisk to combine. Set buttermilk dressing aside.
Using a vegetable peeler, peel broccoli stalk if skin is thick. Halve broccoli lengthwise, then thinly slice crosswise, starting at crown. Toss broccoli, cabbage, scallions, sugar snap peas, 2 tablespoons chives, and reserved buttermilk dressing in a large bowl; season with salt and pepper. Serve slaw topped with remaining 2 tablespoons chives.
DO AHEAD: Slaw can be made 6 hours ahead. Cover and chill.