Boil the rice and let it cool. In the meantime, peel the onion and drain the mushrooms.
Cut the mushrooms into slices. Cut the meat, tangerines, onions and pickles into pieces. Place in a bowl with the corn and the cooled rice. Add the yogurt, mayonnaise, mustard and vinegar and stir everything together. Season to taste with pepper and possibly a little salt.
Chill the lettuce for a few hours, preferably overnight, before eating.