Wash the cucumber thoroughly, rub dry and grate coarsely. Peel onion and garlic and chop finely. Wash the dill, dab dry and chop except for a little bit for garnishing. Mix soured milk, olive oil and lemon juice. Add onion, garlic, dill, grated cucumber and juice. Season to taste with salt and pepper.
Leave to soak for about 30 minutes. Rinse crab meat cold and dab dry. Clean, wash and quarter and seed the tomatoes. Cut the flesh into small cubes. Sprinkle the shrimps and diced tomatoes over the cold dish just before serving. Garnish with dill