With the rack in the middle position, preheat the oven to 200 °C (400 °F).
In a bowl, combine the spices. Brush the roast with the mustard, then coat with spices.
In a large ovenproof skillet, brown the roast in half the butter. Season with salt and pepper. Set aside on a plate.
In the same skillet, brown the onions over high heat in the remaining butter. Place the roast over the onions and bake for about 45 minutes or until a meat thermometer inserted into the centre of the roast reads 48 °C (118 °F) for medium rare doneness. Count about 15 minutes per 454 g/1 lb of meat. Cover with foil. Let stand for 10 minutes at room temperature.
In a bowl, dissolve the cornstarch in the water. Set aside.
In a saucepan, bring the broth, Marsala wine, and molasses to a boil. Let reduce until about 1 litre (4 cups) of liquid remains. Add the cornstarch mixture and cream and bring to a boil, stirring with a whisk. Add the butter and stir until melted. Season with salt and pepper.
Thinly slice the meat and drizzle with the sauce. Serve with the onions, steamed carrots or Braised Celery and Mashed Potatoes with Garlic and Rosemary.
To get ahead, you can brush the roast with the mustard and rub it with the spices the day before. Slice the onions and even prepare the sauce in advance. Cooking the roast should be done at the last moment.