Potato dauphinoiseMary Berry recently celebrated her 90th birthday with royal shout-outs and, per the baking legend, “cake and champagne”.I celebrated it, too, by making some of her very best recipes. The Cordon Bleu-trained chef isn’t afraid of mixing up a classic; she puts cream in her bolognese (which was surprisingly good), makes a “Mexican lasagne” with tortilla wraps, and even whacks Thai curry paste in her tomato soup.Every time I’ve given her twists a go, I’ve been pleasantly surprised. So, I withheld judgment when I read that her potato dauphinoise recipe involved a very un-French ingredient. Mary puts sweet potato in the dish The recipe, shared in