Combine rhubarb, sugar, water, and strawberries in a saucepan.

Bring the mixture to a boil over medium-high heat.

Reduce heat to low and simmer for 15 minutes, or until the fruit is tender, stirring occasionally.

Reduce heat to the lowest setting.

Add lemon juice and marshmallows.

Cook, stirring continuously, until marshmallows are completely melted and incorporated into the mixture.

Remove the saucepan from heat.

Transfer the mixture to a large mixing bowl.

Cover the bowl and refrigerate overnight, or for at least 8 hours, to allow the flavors to meld and the mixture to chill thoroughly.

In a separate chilled mixing bowl, pour in the whipping cream.

Beat the whipping cream with an electric mixer until soft peaks form.

Gently fold the whipped cream into the chilled fruit mixture.

Pour the combined mixture into an ice cream maker.

Churn the ice cream according to the ice cream maker's instructions until it reaches a soft-serve consistency.

Transfer the ice cream to an airtight container.

Freeze the ice cream for at least 4 hours, or until it is firm enough to scoop.