1 Peel shrimp, leaving the tails attached. Place the shells in a sieve, rinse and drain.
2 Slit each shrimp with a small knife along its back and devein. Rinse the shrimp and pat dry.
3 Halve leek lengthwise, rinse well, drain and cut into about 2 cm (approximately 1-inch) slices.
4 Put shrimp shells in a pot. Add water to cover the shells by about 2 cm (approximately 1 inch) and bring to a boil.
5 Place leek and shrimp in a steamer basket and set in the pot. Cover and steam until shrimp are pink and opaque, about 4 minutes.
6 Remove the steamer basket and let leek and shrimp cool. Reserve 3 tablespoons cooking liquid and discard the rest.
7 Rinse tomatoes, wipe dry and cut out stems. Cut tomatoes into wedges.
8 Thoroughly rinse cucumber, wipe dry and cut in half lengthwise. Remove seeds with a spoon and cut cucumber into thin slices.
9 Whisk together mustard, white wine vinegar and reserved cooking liquid in a small bowl and season with salt and pepper. Whisk in olive oil.
10 Rinse dill, shake dry and pluck off fronds.
11 Mix dill with tomatoes, cucumber, leek, shrimp and dressing in a bowl. Let marinate for 15 minutes, season again with salt and pepper and serve.