Preheat oven to 425 degrees. Line 2 baking sheets with parchment paper. In food processor, combine flour, butter, salt, and cheese. Pulse until mixture resembles coarse meal. Add the 5 tablespoons cold water; pulse until the dough just comes together. Shape dough into a disk.
In a small bowl, whisk together egg yolk and remaining 1 teaspoon cold water; set aside. On a lightly floured surface, roll dough 1/4 inch thick. Cut out shapes using fish-shape cookie cutter; transfer to prepared baking sheets. Bring scraps together, reroll the dough, and cut out more shapes. Brush each fish with the reserved egg wash. Bake until golden, about 10 minutes. Transfer to a wire rack, and let cool. Crackers may be kept at room temperature in an airtight container up to 2 days.