In a saucepan, combine rhubarb, strawberries, sugar, and lemon juice. Simmer until the rhubarb and strawberries break down and everything becomes jammy—about 10 minutes.
Add cornstarch/water mixture and bring to a light boil until thickened. Mixture will thicken further when cooled.
Using a stick blender, whip until a smooth consistency forms. Allow some bits of fruit to remain. Let it cool and place in the refrigerator over night to get really cold.
In a bowl, whisk together cream, milk, and the strawberry/rhubarb mixture until well combined.
Pour the chilled cream base into your ice cream maker. During the last 5 minutes, add extra bits of strawberries if desired.
Transfer to a container and freeze for 3–4 hours for scoopable perfection.