Clean the Brussels sprouts and cook in salted water for 3-4 minutes. Rinse with cold water, drain and cut in half.
Wash and quarter tomatoes, remove seeds and stalks. Mix the kernels with 1/2 teaspoon salt and drain in a sieve. After 10 minutes, strain through the sieve, halve the tomato quarters again.
Pluck the parsley leaves from the stem. Roughly chop the walnut kernels. Mix the Brussels sprouts, tomatoes, parsley leaves and walnuts together. Mix the tomato stock with vinegar, a pinch of sugar, pepper and oil and pour over the salad.