Preheat oven to 400 degrees.

In the bowl of a food processor, combine the flour, salt, and 1 Tbsp of the sugar for the crust. Add 14 Tbsp cold butter and pulse until the mixture resembles a coarse meal. Pulse in cold water, 1 Tbsp at a time, until the dough comes together and is still a little crumbly. Turn the dough out onto a lightly floured surface and knead gently. Shape into a disk, wrap in plastic, and refrigerate for 20 minutes.

Combine rhubarb, raspberries, 1/2 cup flour, and 2 cups sugar in a bowl; set aside.

Divide dough into 6 pieces and roll each disc out into an 8" circle. Place each circle of dough in a mini cast iron skillet, pressing the dough into the pan and letting the excess dough hang over the edges.

Lightly flour the dough on the bottom of each mini cast iron skillet. Add enough of the rhubarb raspberry filling until it reaches slightly over the top of each skillet. Fold the edges of the pastry shells over the rhubarb filling, leaving tarts open at the center. Gently brush the pastry with melted butter and sprinkle with extra sugar.

Bake for 30 minutes, until golden brown. Allow to cool slightly on cooling racks before serving.