In a saucepan, combine chopped rhubarb, sugar, lemon juice, and cornstarch. Cook over medium heat until rhubarb is tender and mixture thickens, about 10-15 minutes. Let cool.

In a mixing bowl, combine oats, flour, brown sugar, melted butter, and cinnamon. Mix until crumbly. Spread evenly in a baking dish and bake at 350?F (175?C) for 15-20 minutes until golden brown. Let cool.

In a mixing bowl, whisk together heavy cream, whole milk, sugar, vanilla extract, and salt until sugar is dissolved.

Pour the ice cream base into an ice cream maker and churn according to the manufacturer's instructions until it reaches a soft-serve consistency.

Layer the churned ice cream, rhubarb filling, and crumble topping in a container. Swirl gently to combine. Freeze for at least 4 hours before serving.