Brush the mussels vigorously under cold running water, sorting out damaged and open ones. Pull off the hard threads (beard) with a small knife and remove the mussel.
Onions peel, halve and in strips cut. Peel garlic and cut into slices. Peel carrots, wash and quarter them lengthwise. Clean, wash and slice the fennel.
Heat the oil in a large pot. Brown the onions, garlic and vegetables in it. Season with salt and pepper. Add wine and 1⁄4 l water and bring to the boil. Add the mussels and cook covered over a low heat for 8-10 minutes until the mussels open.
Pour the mussels and vegetables onto a sieve and collect the stock.
Pour cream and 1⁄8 l stock into a small pot. Stir in sauce powder, bring to the boil. Beat butter in small pieces over low heat.
Remove the mussels from their shells and place them with the vegetables in an ovenproof dish. Spread the sauce on top. Gratinate under the preheated grill or in the oven at high heat for about 5 minutes. Wash the chervil, shake dry and remove the leaves.
Arrange mussels. Sprinkle with chervil. Serve with ciabatta.
Drink tip: white wine, e.g. a Sauvignon blanc.