Whisk together the mayo, dill, sugar, vinegar, mustard, garlic powder, pepper and salt in a large bowl.
Remove the stem from the broccoli and slice thinly, then add to the bowl. Cut the crown of the broccoli into small florets and add to the bowl, along with the cabbage and fennel. Toss until the vegetables are evenly coated in the dressing. Garnish with the fennel fronds.
Serve immediately or store refrigerated in an airtight container. This recipe tastes even better the next day!