Wash and peel the onion and parsley root and cut into small cubes. Rinse the soup meat. Melt some butter in the pressure cooker, sauté the root vegetables in it, add the spices and the water.
Close the pressure cooker, heat the pot and let it cook under steam for about 20 minutes. Rinse the chives, dab dry and cut into rolls.
In the meantime, boil the potatoes, then strain and peel them.
For the sauce, mix the egg yolks, butter and whipping cream in a bain-marie. When the butter has melted, whisk vigorously until the sauce has thickened a little.
Add the horseradish, gradually adding the juice from the beef and keep whisking. Season to taste with lemon juice and freshly ground white pepper.
Pour the sauce onto a warmed plate, arrange the slices of meat with the potatoes on top and garnish with the chive rolls.