Julienne cucumber and carrot finely.
Slice onion thinly.
In a bowl, mix 2 tablespoons of water, 2 tablespoons of vinegar, 1 tablespoon of soy sauce, 1 teaspoon of sugar, 1 teaspoon of mustard, and 1 teaspoon of sesame oil to make the sauce.
Combine the vegetables and sauce well, then serve on a plate.