This recipe is from Eating In by Philip Johnson.
Boil or steam the potatoes in their skins until tender. While still warm, peel and cut in half lengthways, then gently toss through the black truffle paste, truffle oil and enough extra virgin olive oil to coat. Season with sea salt and black pepper. Cool, then cover and refrigerate overnight.
Rinse the sweetbreads under cold water for several minutes. Soak overnight in cold salted water - use 100g (¾ cup) salt for every 1 litre (4 cups) water.
Remove the sweetbreads from the water and gently rinse in fresh cold water.
Put the sweetbreads, onion, thyme and peppercorns in a saucepan and completely cover with the milk and chicken stock. Over low heat, bring just before a simmer, then remove from the heat and set aside to cool completely. Strain the sweetbreads, discarding the cooking liquid and aromatics. Clean the outer membrane from the sweetbreads then cut into 1cm pieces.
To make the sauce soubise, melt the butter in a heavy-based saucepan over medium heat. Add the onion and sauté, without colour, until they just begin to soften. Add the eschalot and continue to cook for 2 minutes. Season with sea salt and black pepper, add the stock and diced butter and continue cooking until tender. Transfer to a food processor and purée until smooth. Check the seasoning, then strain through a fine sieve. Set aside and keep warm.
Preheat the oven to 220°C (425°F/Gas 7).
Brush the eye fillets with olive oil and season with sea salt and black pepper. Heat a large heavy-based frying pan over high heat and seal the beef in the hot pan for 2-3 minutes each side. Transfer to the oven and cook for 5-6 minutes, or until pink (medium). Remove from the oven and set aside to rest for 5 minutes.
Spread the potatoes over a baking tray lined with baking paper and warm through in the hot oven for several minutes.
Meanwhile, heat a good knob of unsalted butter in a small heavy-based frying pan over medium heat. Lightly dust the sweetbreads with flour. Once the butter begins to foam, add the sweetbreads and cook until crisp. Drain on paper towels.
In a separate frying pan, melt a good knob of butter over medium-high heat. As it begins to foam, add the baby spinach and season with sea salt and black pepper. Gently toss until the spinach just begins to wilt. Remove from the heat and drain on paper towels.
To serve, place a spoonful of sauce soubise in the centre of each serving plate, using the back of the spoon to spread into a circle. Place the wilted spinach in the centre of the soubise, with three potato halves around the outside.
Place the beef on the spinach. Dot the sweetbreads around the plate and drizzle the warm jus over and around.
Black truffle paste is available from selected delicatessens and food emporiums, We prefer to the use the Elle Esse brand.