1. Preheat the oven to 190°C or 375°F.

2. Peel and dice the parsnip and boil until tender in salted water, then drain and set aside.

3. Heat some rapeseed oil in a large pan over medium heat, which is approximately 180°C or 350°F.

4. Add the diced onion to the pan and cook until softened, then add the garlic and cook for another minute.

5. Add the ground beef to the pan, breaking it up with a spoon as it cooks, and once browned, season with salt, pepper, and a pinch of mustard.

6. In a separate pan, sauté the chopped spinach with some rapeseed oil until wilted, and season with salt and pepper to taste.

7. Create a layer of potatoes in a greased 20x30 cm or 8x12 inch baking dish, then top with a layer of the beef mixture, followed by a layer of sautéed spinach, and finish with a layer of crushed tomatoes.

8. Repeat the layering process until all ingredients are used up, finishing with a layer of potatoes on top.

9. In a bowl, mix the grated cheese with the concentrated veal stock, and spread this mixture evenly over the top layer of potatoes.

10. Drizzle the heavy cream over the cheese mixture, and sprinkle with chopped thyme.

11. Cover the dish with aluminum foil and bake for 30 minutes, then remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown.

12. Remove from the oven and let rest for 10 minutes before serving.