For the potato gratin with herb hollandaise, first prepare the gratin. To do this, preheat the oven to 180° C. Grease an oven dish and put the potatoes in it. Mix whipped cream and clear soup and heat on the stove. Remove the pan from the heat and stir in the egg.
Season with salt and pepper and pour over the potatoes. Bake in a hot oven for about 40 to 60 minutes.
Meanwhile, for the herb hollandaise, bring wine, vinegar, onion, peppercorns and a pinch of salt to a boil in a saucepan and reduce by half the liquid while watching.
Strain the remaining liquid through a sieve into a metal bowl. Mix in the yolks and beat in a bain-marie until foamy.
Melt the butter and slowly beat into the egg mixture. Stir in tarragon and chervil leaves. Add chives and basil if desired.
Serve the potato gratin with herb hollandaise hot.