Preheat oven to 400 degrees. Line a rimmed baking sheet with a silpat or parchment paper.

In a small bowl, toss the rhubarb pieces with lemon zest and 1/2 cup sugar. Spread the mixture on the lined baking sheet in an even layer. Roast in the oven for 15 minutes. Remove from the oven and, using a blender or immersion blender, puree the roasted rhubarb. Set in the refrigerator while you prepare the ice cream base.

In a medium saucepan, stir together heavy cream, whole milk, and 3/4 cup sugar over medium-high heat. Bring the mixture to a low boil.

In a small bowl, combine egg yolks and remaining 1/4 cup sugar. Whisk together until smooth. Slowly add 1/4 cup of the heavy cream mixture to the egg mixture, whisking quickly to incorporate. Once well combined, add the egg mixture to the heavy cream mixture. Cook over medium heat until the mixture is thick and coats the back of a metal spoon.

Remove from heat. Stir in the roasted rhubarb puree. Add the custard to a large bowl and cover with plastic wrap, making sure the plastic wrap touches the surface of the custard to prevent a film from forming. Chill completely.

Add the custard mixture to your ice cream machine. Churn the ice cream according to the manufacturer’s instructions. Chill overnight or for a minimum of 7-8 hours before serving.