This strawberry rhubarb sheet cake recipe is a simple and easy dessert that is perfect for feeding a crowd. The cake is made with a moist and fluffy vanilla cake base, topped with a layer of fresh strawberries and rhubarb, and finished with a crumbly streusel topping. The secret to this recipe is to use a large sheet pan to create a thin and even cake. Serve this cake at room temperature with a dollop of whipped cream for a delicious dessert.