The beauty here is roasting the duck with the broccoli. As the duck fat is rendered you end up frying your broccoli. And a little broccoli fried in duck fat never hurt anybody. Use a tin casserole and you're golden. —Hi, I'm Brian.
Next, using a sharp knife, score the duck breast in a cross-hatch pattern creating diamond shapes about 1/2″ square. Be sure not to cut all the way to the meat. Then rub the duck with the mustard and soy sauce.
Arrange the duck in the middle of a steel casserole dish, fat side up. If you don’t have a steel casserole dish go ahead and use ceramic or glass, but steel will heat up a little better and brown everything nicely from below. Place the broccoli florets on either side of the duck and season with salt and pepper.
Place the casserole dish in the oven and roast for about 20 minutes for medium rare, but check after 15. Depending on the size of the breast it may take longer or shorter. By the end the duck should be golden on the outside and slightly pink in the middle. If you prefer well done, stir the broccoli partway through to coat it in duck fat, which will prevent too much blackening from a prolonged cook time.