Preparation

Make the pastry dough:

• Work together the flour, cornmeal, brown sugar, butter, and salt with your hands until it is mostly combined, with some small lumps of butter remaining. Stir in 2 Tbs. of the water with a fork. Press a small handful of dough together: if it looks powdery and does not come together, stir in the remaining 1 Tbs. water. Transfer the dough to a sheet of plastic wrap. Using the edge of the plastic, fold the dough over on itself, pressing until it comes together. Form the dough into a disk, wrap completely in the plastic, and chill for 1 hour.
• Preheat the oven to 425°F, with a rack in the middle.

Fill and bake the crostata:

• Spread the sour cream evenly over the bottom of the crust. Toss the rhubarb with the sugar and lemon zest, then spread the fruit evenly over the sour cream. Fold the border of dough up and over the edge of the fruit.
• Bake the crostata until the crust is golden, the filling is bubbling, and the rhubarb has started to brown, 45 to 50 minutes. Cool on a rack and serve warm or at room temperature.