In a small saucepan, mix sugar and cornstarch. Stir in water. Add 1 cup strawberries; mash mixture. Bring to a boil; cook and stir until thickened, 1-2 minutes. Remove from heat; stir in lime zest. Transfer to a small bowl; refrigerate, covered, until chilled.
Cut sponge cakes crosswise in half; trim the cakes to fit in the bottoms of 4 wide-mouth half-pint canning jars. In a small bowl, mix blueberries and remaining strawberries; spoon over cakes. Top with sauce. If desired, serve with whipped topping.
Test Kitchen Tips
Shortcakes (homemade or store-bought) can be swapped for sponge cake here if that floats your boat. Crumble large pieces into the bottom of the canning jars before topping with fruit and sauce.
A little vanilla extract folded into the whipped topping—or fresh whipped cream, if you prefer—is delicious.