Chop the cauliflower into florets (watch the video below). Slice the onion. Core and chop the apples into bite-sized pieces, but leave the skins on. Coarsely chop the dill.
Place the cauliflower, red onion, and apples into a large bowl and mix together with the olive oil, kosher salt and plenty of fresh ground pepper.
Line a baking sheet with parchment paper. Place the chopped vegetables on the baking sheet, then roast for 30 to 40 minutes until the cauliflower is tender and slightly browned, stirring twice during the baking time. Remove from the oven, stir in the dill, and place in a serving dish. Serve immediately.