**Prepare the Rhubarb Compote:** In a medium saucepan, combine chopped rhubarb, sugar, water, and lemon zest. Cook over medium heat, stirring occasionally, until rhubarb is softened and has released its juices, about 10-15 minutes. Mash some of the rhubarb pieces with a fork for a smoother consistency. Let cool completely.

**Prepare the Oat Crumble:** Preheat oven to 350°F (175°C). In a medium bowl, whisk together flour, oats, brown sugar, and cinnamon. Pour in the melted butter and stir until the mixture is crumbly and evenly moistened. Spread the crumble mixture onto a baking sheet lined with parchment paper. Bake for 12-15 minutes, or until golden brown and crisp. Let cool completely, then break up any large clumps.

**Make the Ice Cream Base:** In a medium saucepan, whisk together heavy cream, milk, and 0.5 cup of the granulated sugar. Heat over medium heat until just simmering (do not boil). Remove from heat.

In a separate bowl, whisk the egg yolks with the remaining 0.25 cup granulated sugar until pale yellow and slightly thickened. Gradually ladle about 1 cup of the warm cream mixture into the egg yolk mixture, whisking constantly to temper the eggs. Pour this tempered egg mixture back into the saucepan with the remaining cream mixture.

Cook the custard mixture over low heat, stirring constantly with a wooden spoon or spatula, until it thickens enough to coat the back of the spoon (about 175°F/80°C). Do not let it boil. Strain the custard through a fine-mesh sieve into a clean bowl. Stir in the vanilla extract and salt. Cover with plastic wrap, pressing it directly onto the surface to prevent a skin from forming. Chill in the refrigerator for at least 4 hours, or preferably overnight.

**Churn the Ice Cream:** Pour the chilled custard base into your ice cream maker and churn according to the manufacturer's instructions until it reaches a soft-serve consistency.

**Assemble the Ice Cream:** While the ice cream is churning, prepare a freezer-safe container. Once the ice cream is churned, gently fold in half of the cooled rhubarb compote and half of the cooled oat crumble. Layer the ice cream into the container, swirling in the remaining rhubarb compote and oat crumble as you go. Freeze for at least 4-6 hours, or until firm.