Mine is less of a dish and more of a bake. I am pretty good at making bread, and my mum always looks forward to my wholemeal or rye loaves. My mum's signature dish is her rice pudding. No one can do rice pudding liker her. It is very simple, with just rice, milk, and sugar, but it does not need any other fancy ingredients.
Out of our entire family, there is only one person we really consider to have a true signature dish though, and that i my grandpa's sponge cake. It is a Boxing Day must. Unlike most people that need to bake three separate cakes, my grandpa's sponge rises so high, he has to slice it into three. He then sandwiches them together with apricot jam, smothers it with cream, and decorates with chocolate drops and cherries. Hugh as a house, but light as feather.