For the asparagus purée, cook the sliced asparagus & spinach (reserving some to sauté) in a pan of boiling salted water for 3-4 minutes, or until soft. Drain, and then squeeze out any excess water. Place into a food processor with the butter and blend until well combined, then add the cream and blend until smooth. Season with salt, and then pass the mixture through a sieve into a small bowl. Set aside.
For the onion ash, preheat an oven to 60C/140F/Gas ¼. Place the onions into a hot, dry, heavy based pan; cook the onions until black on all sides. Place the onions onto a tray and place into the oven overnight or until dry and crisp. Once fully dried, place the onions into a spice grinder, or small food processor, and blend to a fine powder.
For the sweetbreads, season the sweetbreads with salt and freshly ground black pepper. Place the duck fat into a pan and heat up to 60C/140F using a thermometer, then add the sweetbreads and cook for one hour, keeping an eye on the temperature. Once cooked, remove the sweetbreads from the pan with a slotted spoon and drain off any excess duck fat. Place the sweetbreads into a hot pan with a little oil and brown on all sides, add a knob of butter and baste the sweetbreads with the butter.
For the griddled asparagus blanch in boiling salted water until just tender then drain and place on a smoking griddle pan and chargrill on all sides. Meanwhile sauté off some spinach in butter and drain
Peel the potato and put through a Japanese turning machine fry in a deep fat fryer at 180C until golden