STRAWBERRY-RHUBARB SAUCE: In a saucepan, cook the rhubarb and strawberries over medium-high heat, stirring often, until they begin to release their juices. Whisk together the sugar and the corn-starch, then add it to the saucepan. Bring to a boil, stirring constantly, then reduce heat to low and let simmer until the sauce begins to thicken. Stir in the lemon juice and lemon zest and cook for one for minute. Remove from heat, transfer to a bowl, and refrigerate until cool.