Prepare the Cabbage: Preheat your oven to 425°F (220°C). Place a sheet of parchment paper on a baking tray or give it a light coat of oil. Remove any damaged outer leaves from the cabbage and cut it into 1-inch thick wedges, keeping the core intact to hold the wedges together. Arrange the cabbage and red onion wedges on the prepared baking sheet.

Season and Roast: Melt the butter and combine it with the olive oil in a small bowl. Brush the mixture generously over the cabbage and onions, ensuring even coverage. Roast in the preheated oven for 25-30 minutes, flipping halfway through, until the edges of the cabbage are golden brown and crispy.

Make the Hot Honey: While the vegetables are roasting, combine the honey, red pepper flakes, Dijon mustard, and apple cider vinegar in a small saucepan. Heat over low heat, stirring until fully combined and warmed through. Remove from heat and set aside.

Serve: Once the cabbage and onions are done, transfer them to a serving platter. Drizzle the hot honey sauce over the roasted vegetables just before serving. Serve warm as a side dish or pair it with your favorite main course.