Preheat the oven to 400 degrees F (200 degrees C). Butter a baking pan.
Cover the bottom of the prepared baking pan with cabbage. Arrange pancetta over the cabbage. Top with cauliflower and couscous.
Melt butter in a medium saucepan over medium heat. Whisk in flour. Reduce heat to medium-low; whisk in milk. Cook until mixture thickens, about 2 minutes. Whisk in oregano and salt. Add Swiss cheese, black pepper, and cayenne pepper. Whisk until smooth. Pour sauce over the cauliflower in the baking pan. Sprinkle Parmesan cheese and bread crumbs on top.
Bake in the preheated oven for 20 minutes. Reduce oven temperature to 350 degrees F (175 degrees C). Continue baking until cauliflower is tender, about 20 minutes more. Tent with aluminum foil if top is browning too quickly. Transfer gratin to a wire rack; let cool, about 10 minutes.