Give the meat a bit of salt and plenty of pepper and vacuum seal it.

Place it in the sous vide/water bath for 3 hours at 58 degrees Celsius (136 degrees Fahrenheit).

Pat the meat dry and place it in a hot pan with oil and 10 grams of butter.

The browned butter is poured over the meat when serving.

Cut the half celeriac into small cubes of about 1x2 cm. Boil them in 1/3 water, 1/3 milk, and 1/3 cream for 18-22 minutes, until completely tender. Lift them out with a slotted spoon and place them in your blender/container that you can use an immersion blender in. Add 10 grams of butter, a little vinegar, salt, and pepper, and blend it with some of the liquid it was boiled in.

Gradually add the liquid until it is a nice and smooth puree.

 

Cut the potatoes into cubes (this is not necessary, but it looks great) and boil them for 7 minutes in lightly salted water.

Drain the water through a large sieve, shaking it well so the potatoes get cracks and more surface area that can become crispy. Let them dry for 20-30 minutes and then place them in the oven tossed with oil, salt, and pepper. 

Roast them for 40 minutes at 200 degrees Celsius (392 degrees Fahrenheit) with fan and the potatoes are ready to be served.