Chop the rhubarb – 1 pound rhubarb stalks will yield about 3 cups chopped rhubarb.
Put the chopped rhubarb, sugar, cornstarch, and cardamom in a small saucepan. Cook over medium heat, stirring occasionally, until the rhubarb breaks down: about 5 minutes.
Set aside to cool, then refrigerate at least 2 hours and up to 3 days.
TO ASSEMBLE THE TARTS
Divide the chilled rhubarb compote among the tart shells. Top with whipped cream, add a mint leaf if desired.