Remove the skin from the onion and cut it. Rinse the cauliflower, cut into roses and cook in salted water. Chop the chervil, chives, tarragon and basil. Rinse the potatoes, remove the skin and cut into slices.
Preheat the oven to 180 degrees Celsius (convection oven: 160 degrees Celsius /gas oven: level 3).
Grease a gratin dish and layer the potatoes in it. Heat the whipped cream and clear soup, remove from the heat and mix the egg well in it. Season with salt and pepper and pour over the potatoes. Cook in the heated oven for 35 to 40 minutes.
Boil the onion, vinegar, wine and peppercorns and reduce by half the liquid. Remove the reduction from the fire, strain through a sieve into a whipping kettle, add yolks and beat over a water bath until firm.
Melt butter and gently fold into egg mixture. Stir in tarragon and chervil If desired, also add chives and basil.
Arrange cauliflower florets with herb hollandaise on hot plates and serve with potato gratin.
Our tip: If you like to cook with fresh herbs, it is best to get kitchen herbs in pots – so you always have everything ready!