Clean iceberg lettuce, cut into quarters, remove the hard stalk and cut the quarters into strips. Clean and peel carrots and cut or slice into fine sticks. Drain corn kernels.
Clean spring onions, cut the white and light green into fine rings. Clean tomatoes, halve and cut halves into thin slices. Mix sour cream, milk, creamed horseradish, lemon zest, lemon juice, some salt, pepper and sugar. Cut cress from the bed and mix in 2/3 of it. Season dressing to taste.
Place lettuce, carrots, tomatoes, corn and spring onions in a bowl, spread the dressing over them and sprinkle the remaining cress on top.