Step 1 Combine the rhubarb, strawberries and sugar in a small saucepan over low heat until the fruit softens and breaks down, stirring occasionally, about 15 minutes. Stir the cornstarch and water together in a small bowl. Add to the fruit and continue cooking on low heat, stirring often, until the fruit thickens, about 5 more minutes. Cool completely and store covered in an airtight container in the refrigerator. It will stay fresh up to a week.