Rinse and drain the prawns. Wash the lemon hot, dry and finely grate half of the peel. Halve the lemon, squeeze the grated half. Wash the dill and shake dry.
Pluck flags from the stems and cut them finely. Mix lemon zest, 1 tbsp. lemon juice, dill, oil and prawns. Marinate for at least 30 minutes.
Clean, wash and cut the radishes into small cubes. Wash the cucumber, halve lengthwise and remove seeds. Dice the cucumber finely as well.
For the cocktail sauce, mix mayonnaise, ketchup and cognac. Season to taste with salt. Divide the prawns and marinade into four glasses and add some sauce to each. Spread the cucumber and radishes on top.
Pour the rest of the sauce over it. Grissini sticks taste good with it.