CUT IN BUTTER: Combine flour, corn starch, sugar, and salt in food processor with three 1-second pulses. Scatter butter pieces over flour, pulse to cut butter into flour until butter pieces are size of large pebbles, about 1/2 inch, about six 1-second pulses.
ADD WATER: Sprinkle 1 tablespoon water over mixture and pulse once quickly to combine; repeat, adding water 1 tablespoon at a time and pulsing, until dough begins to form small curds that hold together when pinched with fingers (dough should look crumbly and should not form cohesive ball).
FORM MOUND: Empty dough onto work surface and gather into mound about 12 inches long and 5 inches wide.
FRAISAGE AND CHILL: Starting at farthest end, use heel of hand to smear small amount of dough against counter, pushing firmly down and away from you, to create separate pile of dough (flattened pieces of dough should look shaggy). Continue process until all dough has been worked. Gather dough into rough 12 by 5-inch mound and repeat smearing process. Dough will not have to be smeared as much as first time and should form cohesive ball once entire portion is worked. Form dough into 4-inch square, wrap in plastic, and refrigerate until cold and firm but still malleable, 30 minutes to 1 hour. If the dough has chilled longer than 1 hour, let it stand at room temperature for 15 to 20 minutes to soften.
MAKE THE FILLING: If the rhubarb stalks are more than 1-inch thick, cut them in half lengthwise. Cut the stalks into 1-inch long pieces. In a large bowl, gently toss the rhubarb and raspberries with the flour and sugar. Let stand until moist, 5 to 10 minutes.
ROLL DOUGH: Place dough on floured 16-inch piece of parchment paper and dust with more flour. Roll dough into a 14-inch round, about 1/8 inch thick. If the dough becomes soft and sticky while being rolled, transfer it to a baking sheet and refrigerate it for 10 to 15 minutes.
HEAT THE OVEN TO 400°F: Spread the fruit in the center of the chilled pastry, leaving a 2-inch margin around the edge. Carefully fold the edge of the dough over the fruit, pleating it as you go. Brush the edge of the dough with melted butter and sprinkle with sugar.
BAKE: Until the pastry is golden brown and the fruit is soft and bubbling, 45 to 55 minutes. Check the bottom of the galette halfway through baking-it should be a light golden brown. If it is darker, reduce the oven temperature to 375°F.
Set on a rack to cool slightly. Transfer to cutting board. Cut in half lengthwise and then crosswise into individual portions; serve with vanilla ice cream, whipped cream, or creme fraiche.