Start the crust: In a food processor, pulse together the flours, salt, and celery seeds a few times just to mix. Add the cubed butter and pulse again, until it breaks down into pea-size pieces and the mixture resembles a coarse meal. With the motor running, slowly trickle the ice water into the food processor, just until the dough comes together. Don’t add too much water or overmix the dough; it should take 30 seconds, tops. Test it by squeezing a bit between your thumb and index finger; if it’s too crumbly, add 1 to 2 teaspoons more ice water, pulsing the machine this time to quickly incorporate the water. Test the dough again, continuing until it’s the right consistency.