Spanish Olive Oil-poached Sturgeon, Marinated Red Cabbage, Membrillo, and Celery Root Cream
For the Spanish Olive Oil-poached Sturgeon:
Mix the salt, sugar, and lemon zest together. Put the sturgeon fillet on a metal baking sheet covered with parchment paper, and sprinkle the salt mixture over it, turning it to coat evenly. Cure the sturgeon at room temperature for 1 hour, then rinse the fish under cold water to remove the salt mixture from its surface.
Meanwhile, in a wide Dutch oven or saucepan big enough to fit the fillet, bring 3 inches of water up to a simmer. Put the olive oil into a shallow baking pan or casserole dish. Slip the sturgeon into the simmering water, and reduce the heat to medium-low. Poach the fish for 15 minutes, or until the sturgeon is just cooked through. Remove it from the water with a slotted spoon or fish spatula, and put it into the olive oil. If the olive oil does not completely cover the fillet, add more. The sturgeon can stay submerged in oil for over one month in the refrigerator.
For the Marinated Red Cabbage:
Toss the cabbage in the vinegar, olive oil, sugar, and salt.
For the Membrillo:
Put the membrillo and water together in a blender. Purée on high, stopping the blender to scrape down the sides. Add more water if the purée is too thick to blend evenly. Reserve in a squeeze bottle.
For the Celery Root Cream:
Put the celery root, water, and milk in a saucepot and bring to a simmer over high heat. Reduce the heat to medium low and cook the celery root until it is very tender, about 40 minutes. Remove the pot from the heat, and using a slotted spoon, transfer the celery root pieces to a blender. Only fill the blender half way, and repeat the following procedure in batches if necessary. Add enough cooking liquid to the blender to come up just under the top of the celery root. Purée on high, periodically stopping the blender to scrape the sides down. Add the butter, truffle oil, and salt to the celery root in the blender and blend. Reserve in a squeeze bottle.
To Assemble and Serve:
On 8 small salad plates, put dots of both the Celery Root Cream and the Membrillo. Next, plate a small pile of the Marinated Red Cabbage in the center of the plate. Remove the Spanish Olive Oil-poached Sturgeon from the oil, and slice into even slices. Fan the slices out on top of the Cabbage. Finish with the sea salt and olive oil.