Clean and wash salads, dab dry and pluck into bite-sized pieces. Drain the corn in a sieve. Wash the cucumber and cut into thin slices. Wash, clean and slice tomatoes.
Finely chop the walnuts. Mix vinegar, 2 tablespoons of water and mustard, season with salt and pepper. Gradually add the oil. Add the chopped nuts. Mix all ingredients in a bowl just before serving and arrange on a platter.