In a saucepan, heat the milk and lemon peel to a simmer then turn off the heat and allow to infuse 10 minutes. Meanwhile, whisk the yolks with the sugar and salt till very light in color and fluffy. Whisk in the hot milk and return the mixture to the heat. Cook carefully until it thickens then immediately add the pistachio paste whisking well. Pour the custard base into a new container and cool it in an ice bath. Turn as per manufacturer's instructions.