Start by prepping the peaches. Rinse them, halve them, pit them, quarter them, then chop. Place in a large bowl with the lemon zest, lemon juice, and 50 grams of the sugar. Stir gently, cover with plastic wrap, and place in the fridge overnight.
Make the ice cream base by bringing the milk and cream, plus a pinch of salt, to a simmer, stirring now and then. Meanwhile, start whisking the egg yolks with the rest of the sugar. Temper the egg yolks by pouring some of the warm milk mixture in, whisking furiously, then slowly adding the rest (this prevents the eggs from scrambling).
Pour the mixture back into the pot and stir constantly, over low heat (I got impatient and put it on medium), with a heat-proof rubber spatula, until the temperature reaches 180 degrees F. Immediately pour into a soup container, let cool a bit, then cover and refrigerate overnight.
The next day, pour the custard over the peaches and use a stick blender to blitz for two minutes, until there are only small flecks of peach skin visible in the mix. Press the peach custard through a fine-mesh sieve into a clean container (I used a large measuring cup), squeezing hard to extract as much smooth custard as possible. Discard the peach skin.
Pour the mixture into your ice cream machine and churn for about 20 to 25 minutes, until frozen and the texture of whipped cream.
Transfer whatever you can to a container and freeze and whatever's left you should eat right away. It'll be the highlight of your day.