For the gratin
Preheat the oven to 180'C.
Peel the potatoes and slice into 2mm thick slices using a mandoline or sharp knife. Peel and finely dice the garlic. Peel and finely grate the horseradish. Roughly chop the parsley.
In a large saucepan, bring the milk and double cream to the boil. Stir in the garlic and grated horseradish to the pan and cook for one minute. Add the potatoes to the pan and season with salt. Bring back to the boil and then add half of the the chopped parsley. Remove the potatoes from the pan using a slotted spoon and place in a deep baking tray. Spoon over any remaining cream and place the tray in the oven and cook for 40 minutes until the top is golden brown. Check the potatoes are cooked using a skewer.For the parsnips
Peel the parsnips and cut lengthways into quarters.
Place in a baking tray with the seasoning, thyme and oil. Roast in the oven for 45 minutes until soft and caramelised on the edges. Pour on the honey, making sure all of the parsnips are coated and then return to the oven for 5 minutes.
For the vinagrette
Soak the raisins in the cognac for 15 minutes. Mix in the honey, balsamic vinegar, remaining parsley and olive oil. Stir to make a vinaigrette. Taste the vinaigrette and adjust with more honey or olive oil to achieve a balanced flavour.For the beef and the foie gras
Cut the foie gras into six pieces approximately 1cm thick.
Keep refrigerated until the steak is cooked.
Season the beef with salt and black pepper. Heat a frying pan until very hot and add a splash of sunflower oil. Sear the steaks on all sides and then transfer to the oven to finish cooking. Depending on the thickness of the steaks and the degree to which you need it cooked this will take between 10 and 15 minutes. Allow the steak to rest for 5 minutes before serving.
Whilst the steak is resting, begin to plate the dish.
Use a stainless steel ring to cut the gratin and then place the gratin servings in the middle of each plate. Drizzle the vinaigrette around the edge of the plate.
Heat a frying pan until hot. Season the foie gras with Maldon sea salt and then sear the foie gras for 15 seconds on each side. Remove from the pan and finish plating the dish.
Place the steak on top of the gratin. Carefully place a few of roasted parsnips on top and then finish the dish with the foie gras.