1. Preheat oven to 180°C (350°F)
2. In a deep skillet, brown roast on all sides in oil and butter. Season with salt and pepper. Remove the meat from the pan and place on a rack in a roasting pan.
3. Add the onions and garlic to the pan and sauté until translucent.
4. Place about 2/3 of the onions and garlic in the roasting pan.
5. Add the cooking cream to the pan with the remaining onions and garlic. Mix well and stir in the wholegrain mustard, nutmeg, thyme and rosemary. Season with salt and pepper. Heat until hot, but not boiling.
6. Place a layer of sliced potatoes in a casserole dish, followed by a layer of sliced sweet potatoes. Repeat layers until all vegetables are used up.
7. Pour the creamy mixture over the vegetables, making sure they are well covered. Use a spatula to ensure that the sauce penetrates between the layers.
8. Sprinkle the grated cheese over the top of the gratin, then add the grated Parmesan for a golden, flavorful crust.
9. Cover the dish with aluminum foil and bake for 1 hour. Remove the foil and continue baking for a further 15 to 20 minutes, or until the top is golden brown and the vegetables are tender.
10. At the same time, bake the roast in the oven until the thermometer reads 55C (130F).
11. Remove the gratin and roast from the oven.
12. Let the gratin rest for about 10 minutes before serving, to allow the flavors to blend and the texture to stabilize. Cover roast with aluminum foil and let stand for about 10 minutes.
13. Just before serving, cut the roast into thin slices and serve with the gratin.

Chef's tips

For a surprising touch, add crushed walnuts (like pecans) to the gratin 10 minutes before the end of cooking for an extra crunch.

Variation: Incorporate fresh spinach or kale into the vegetable layers to add a note of greenery and more nutrients.