In a heavy saucepan, combine the milk, cream, and lemon zest. Place over medium heat and bring the mixture just to a simmer. Remove from heat.
Meanwhile, in the bottom pan of a double boiler (or a pan that a metal bowl can nest inside to create a double-boiler arrangement), pour in water to a depth of about 1 inch. Place over high heat and bring to a simmer, then adjust the heat to keep the water simmering barely. Place a metal bowl in a larger bowl of iced water and set aside.
In the top pan of the double boiler (or a metal bowl that can nest inside the pan of simmering water), combine the egg yolks, sugar, and salt. Beat with a hand mixer at medium-high speed until the mixture is pale yellow and creamy, about 3 minutes. Pour the hot milk-cream mixture through a fine-mesh strainer into a heatproof pitcher. With the mixer running, slowly add the hot milk-cream mixture to the egg mixture and beat until smooth. Place the pan (or nest the bowl) over the simmering water and stir the mixture constantly until it is thick enough to coat the back of a spoon (your finger should leave a trail when you run it across the spoon), about 15 minutes; do not allow to boil.
Pour through a fine-mesh strainer into the bowl set in iced water. Place a piece of plastic wrap directly onto the surface of the custard to prevent a skin from forming and set aside to cool for about 15 minutes. Remove the bowl from the iced water and discard the plastic wrap. Cover tightly, and refrigerate until well chilled, at least 3 hours or, preferably, overnight.
Pour the chilled custard mixture into an ice cream maker and freeze according to the manufacturer's instructions. Serve when the ice cream holds together but is still soft. Or, for a firmer texture, pack the ice cream into a container with a tight-fitting lid and place in a freezer for several hours or for up to several days. Transfer the freezer container to a refrigerator for a few minutes before you plan to serve the ice cream to allow it to reach the proper consistency. (Alternatively, microwave the ice cream for about 20 seconds at high power to soften if frozen hard.) If the ice cream gets icy from long freezer storage, whip it with a whisk just before serving.