Onions peel and chop very finely. Pour shrimps on a sieve, rinse with cold water and drain well on a sieve. Whip 125 g cream with the whisk of the hand mixer. Wash the dill, dab dry, put some flags aside for garnishing. Chop the rest of the dill. Melt butter in a pot, add onions and fry until translucent.
Dust with flour, sweat and gradually deglaze with fish stock, broth and remaining cream, stirring constantly. Bring to the boil while stirring constantly, season to taste with salt, pepper and sugar. Stir in cognac. Add shrimps and bring to the boil again briefly. Serve sprinkled with a dollop of whipped cream and dill. Garnish with dill flags