Heat oven to 400° F. In a food processor, combine the flour, salt, and 1 tablespoon of the sugar. Add the butter and pulse until the mixture resembles coarse meal. Add cold water, 1 tablespoon at a time, and pulse until the dough comes together but is still slightly crumbly (you should use 3 to 4 tablespoons total). Turn the dough onto a lightly floured surface and knead gently. Shape the dough into a disk, wrap it in plastic, and refrigerate for 20 minutes.
Meanwhile, in a large bowl, combine the rhubarb, raspberries, and 1 cup of the remaining sugar; set aside.
On a lightly floured surface, roll the dough into a 14-inch circle. Place it in a large cast-iron skillet, allowing the dough to hang over the edge. Top with the rhubarb mixture. Fold the edge of the dough over the fruit (to create pleats).
Lightly brush the top of the dough with water and sprinkle with the remaining sugar. Bake until golden brown, 50 to 55 minutes. Let cool for 5 to 10 minutes. Slice into wedges and serve with the whipped cream or ice cream (if using).
Simple and delicious! Great way to use your rhubarb!