How to Make It
Heat 1 tablespoon of the butter in a nonstick medium skillet. Add the onion and cook over moderate heat, stirring, until browned, 8 to 10 minutes. Let cool.
In a bowl, mix the flour with the sugar, kosher salt and pepper. Work in the remaining 6 tablespoons of butter, rubbing it into the flour with your fingertips until the mixture resembles coarse meal. Mix in the onion. Stir in 3 tablespoons of the ice water and shape the dough into a ball; if necessary, add more ice water, 1 tablespoon at a time. Divide the dough in half and pat into two 4-inch squares. Wrap each square in plastic and refrigerate until chilled, at least 1 hour.
Preheat the oven to 350°. Line 2 large baking sheets with parchment paper. On a lightly floured surface, roll out 1 dough square to a 12-by-9-inch rectangle, about 1/8-inch thick. Cut the dough into 3-inch squares and transfer to 1 of the sheet pans. Brush with some of the egg-white wash and sprinkle with half of the cumin seeds and sea salt. Bake for about 20 minutes, or until the crackers are golden brown and crisp. Let cool completely on a rack. Meanwhile, repeat with the remaining dough, egg-white wash, cumin seeds and sea salt.