How to Make It
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.