Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large bowl, prepare the cake batter according to the cake mix box directions, typically by combining the cake mix, eggs, oil, milk, and vanilla extract. Beat until smooth.

Pour half of the cake batter into the prepared baking pan, spreading evenly.

In a separate medium bowl, gently toss the chopped rhubarb with granulated sugar, flour, and lemon zest until evenly coated. Spread this rhubarb mixture evenly over the cake batter in the pan.

Pour the remaining cake batter over the rhubarb layer, spreading carefully to cover as much as possible. It's okay if some rhubarb peeks through.

Optional Streusel Topping: In a small bowl, combine the flour and both sugars. Cut in the cold butter with your fingers or a pastry blender until the mixture resembles coarse crumbs. Sprinkle evenly over the top of the cake batter.

Bake for 35-45 minutes, or until a wooden skewer inserted into the center of the cake comes out clean and the cake is golden brown.

Let the cake cool in the pan on a wire rack for at least 30 minutes before slicing and serving. This cake is delicious served warm or at room temperature.