Oops, I Dropped the Lemon Tart

Fill a sink or large stainless steel bowl with ice and arrange a second slightly smaller bowl on top of the ice. In a heavy medium saucepan over moderate heat, combine the milk, cream, sugar, lemongrass, and lemon zest. Cook, stirring often with a wooden spoon, until an instant-read thermometer registers 160 °F. Pour through a fine-mesh sieve into the stainless-steel bowl set over ice. Let the mixture cool, whisking occasionally, until chilled. Process custard in an ice cream maker then transfer to an airtight container and freeze until hard, at least 4 hours and up to 1 week.