Using buckwheat flour in these incredibly crisp and buttery crackers gives them an appealing, earthy flavor. They're delightful with lemony crème fraîche and an assortment of smoked fish. More Hors d'Oeuvres
In a large bowl, mix the all-purpose and buckwheat flours with the fine salt. Using a pastry cutter or 2 knives, cut in the butter until it resembles small peas. Sprinkle with the ice water and press gently with your hands to form a dough. Flatten the dough into an 8-by-6-inch rectangle, wrap in plastic and refrigerate for 1 hour.
Preheat the oven to 400°. Line a large rimmed baking sheet with parchment paper. Cut the dough into 4 equal rectangles. Working with 1 rectangle at a time and keeping the rest refrigerated, roll out the dough on a lightly floured surface to a 10-by-12-inch rectangle, about 1/16 inch thick. Transfer to the prepared baking sheet. Brush with the beaten egg and sprinkle with sea salt. Let stand for 3 minutes to let the egg wash dry. With a pastry wheel, cut the rectangle into 2-inch squares. Separate the squares, leaving about 1/2 inch between them. Top the squares with another sheet of parchment paper and another baking sheet. Bake for 25 minutes, until the crackers are golden brown. Transfer to a rack to cool. Repeat with the remaining dough, egg wash and salt.
In a small bowl, mix the crème fraîche, lemon juice and pepper. Arrange the smoked fish, shrimp and onions on a platter. Serve with the crackers and crème fraîche.
The crackers can be stored in an airtight container for 3 days. The crème fraîche mixture can be refrigerated overnight.