Pour the milk, or milk and cream, half of the sugar and, if using, the vanilla pod or lemon zest, into a saucepan and bring the milk nearly to a boil. When the very first bubbles begin to appear, take it immediately off the heat and let it cool. (If using the optional vanilla or lemon zest, you can cover the saucepan, which will slow down the cooling process and prolong the steeping process, thus drawing out more of their flavor.)

When the milk has cooled, put the egg yolks into an electric mixer together with the other half of the sugar. Beat at a high setting until the egg yolks and sugar have been totally combined and developed a creamy quality. You will be able to see ripples as the mixer turns.

Lower the setting and slowly add your cooled milk (from which you will have removed the vanilla or lemon zest if using) in a steady stream. Mix until everything is well amalgamated.

Transfer the mixture back into the saucepan. Heat the mixture gently until the eggs thicken it into a thin custard—thick enough to coat a spoon lightly—but not so much that the egg curdles. Be attentive, increasing the heat very gradually from low to medium or medium-high, stirring constantly until you begin to 'feel' the mixture beginning to thicken, and then lowering the heat immediately to low. If you fee the mixture is getting too hot, you can add a bit of cold milk to the saucepan to avoid curdling.

Once the mixture has thickened nicely, remove it from the heat, pour it into a bowl and chill in the fridge for about an hour. Stir from time to time to prevent a film from forming on the surface of the mixture.

Pour the batter into an ice cream maker and proceed to churn per the instructions that came with the machine. Typically, you will churn the batter for 20-30 minutes, which results in a 'soft serve' texture. If you want a firmer gelato, freeze it for an hour or two before serving.