Bake a tangy Rhubarb Upside Down Cake with caramelized rhubarb and lemon-scented cake.
Preheat the oven to 350°F. While the oven warms, chop 2–3 medium rhubarb stalks into roughly 1/2-inch pieces and toss them with 1 1/2 tablespoons of all-purpose flour in a small bowl so the fruit releases less liquid and the pieces stay discrete while baking. This step is quick and tidy — the flour lightly dusts the rhubarb, giving the stalks a slightly matte, slightly powdery surface that will later cling to the caramelized sugar for a glossy finish.
Pour 1/4 cup melted butter into a 9" round pan so it pools evenly across the base, then scatter 1/2 cup packed brown sugar over the butter so it forms a damp, grainy amber carpet. Arrange the floured rhubarb pieces on top in an intentional pattern (concentric arcs or a loose mosaic) so each piece sits partly embedded in the sugar-butter mix; the rhubarb should look wet, ruby-red, and ready to caramelize. Set the prepared pan aside while you make the batter.
In a medium bowl combine 3/4 cup granulated sugar with 1/4 cup melted butter until glossy and slightly thickened, then whisk in 1 beaten egg and the juice of one lemon. In a separate small bowl whisk together 1 1/2 cups all-purpose flour, 2 teaspoons baking powder, 1 tablespoon lemon zest, and a pinch of salt. Add the dry mixture to the butter-egg mixture alternately with 2/3 cup milk, stirring just until the batter is homogenous—a pale yellow, ribboning batter with a soft, slightly glossy surface and a few small air pockets. Spread this batter gently and evenly over the rhubarb-sugar layer in the prepared 9" round pan so the red pieces peek through in places and the surface is smooth but not overworked.

Bake the assembled pan 32–36 minutes until the cake surface is pale golden at the edges and a toothpick inserted into the center comes out clean; the caramelized sugar beneath will bubble quietly and darken to a deep amber. Let the cake cool in the pan 15–20 minutes so the sugar firms slightly, then run a knife around the edge and invert the cake onto a platter so the glossy, ruby rhubarb becomes the top. Slice and serve warm — the slice should show a moist, pale-yellow crumb contrasting with jewel-like chunks of rhubarb; finish with a scoop of ice cream or a dollop of whipped cream if you like.
