1. In a medium saucepan, heat milk and lemon zest. Bring to a gentle boil, cover, and remove from heat. Steep for 30 minutes.

2. In a small saucepan, cook juice over medium-low heat until reduced by three-quarters, 30 to 40 minutes. Allow syrup to cool completely.

3. Combine egg yolks and sugar in the bowl of an electric mixer. Beat at medium-high speed until very thick and pale yellow, 3 to 5 minutes. Meanwhile, return milk to a simmer.

4. Add half the milk to egg-yolk mixture and whisk until blended. Stir into remaining milk and cook over low heat, stirring constantly, until mixture is thick enough to coat a spoon.

5. Remove from heat and immediately stir in cream. Pass mixture through a strainer into a medium bowl set over an ice bath to chill. Stir in lemon syrup, then freeze in an ice-cream maker according to manufacturer's instructions. Store in a plastic container.