Prepare the bone marrow:

1. Place the bone marrow halves in a container of cold water in the refrigerator. Change the water every 12 hours for 48–72 hours, until the marrow is fully bled and pale.
2. Transfer the marrow to a pot, cover with fresh cold water, and bring to the brink of a boil over medium-high heat. Remove from heat just before it boils.
3. Let the marrow cool in the cooking water, then remove, trim excess surface fat, and store in an airtight container in the refrigerator until needed.

Make the soufflé potatoes:

1. Square off the potatoes and wash them well. Using a mandoline or slicing machine set to 2–3 mm, slice into thin, even sheets. Cut the sheets into 3 × 4.5 cm rectangles.
2. Spread the potato rectangles on a tray lined with absorbent paper and pat dry thoroughly.
3. Heat sunflower oil to 130–140°C (266–284°F) in a deep, wide pot. Fry the potato rectangles in batches, stirring gently, for 6–8 minutes, until they begin to blister and swell.
4. Transfer immediately to a second vessel of oil heated to 190°C (374°F). Fry until fully puffed and golden. Remove and drain. Reserve at room temperature.

Make the main sauce:

1. Combine both mustards, the brown sauce, Worcestershire sauce, brandy, and Tabasco in a bowl. Whisk until smooth and homogeneous. Portion 16 g of sauce per tartare serving.

Prepare the egg yolk sauce:

1. Separate the yolks from the whites. Press the yolks through a fine-mesh sieve into a clean bowl. Cover and refrigerate until plating.

Assemble and serve:

1. Roast the bone marrow halves cut-side up until hot through the center and the fat begins to bubble, about 10–15 minutes. Season with Maldon salt and freshly ground pepper.
2. Return the soufflé potatoes to the 190°C (374°F) oil briefly to crisp and dehydrate them. Drain and season with salt.
3. Cut the beef tenderloin into very small, even cubes. Season with salt and pepper, then fold in 16 g of the main sauce per portion. Mix gently but thoroughly.
4. Spoon the steak tartare directly onto the roasted bone marrow.
5. Add a line of tobiko alongside the meat. Drizzle with sieved egg yolk sauce and scatter chopped chives over the top.
6. Arrange 3 soufflé potatoes on top of each serving.
7. Serve the bone marrow pieces standing upright on a bed of coarse salt to keep them stable.