Cook the rice in boiling salted water for about 20 minutes. Drain tuna and tomato peppers separately on a sieve. Cut the tuna and the tomato peppers into small pieces. Mix vinegar, soy sauce, salt and pepper.
Beat the oil under it. Pour rice into a sieve, rinse briefly with cold water and drain. Put all ingredients in a bowl, mix with the marinade and let it stand covered for about 30 minutes.
Clean and wash the endive salad, cut into bite-sized pieces and fold into the salad just before serving. Season to taste again if necessary. Arrange on plates and garnish with lemon slices.