Buckwheat Crackers with Smoked Fish

Name: Buckwheat Crackers with Smoked Fish
Ingredients: 3 cups all-purpose flour, plus more for dusting; 1/4 cup buckwheat flour; 1 teaspoon fine salt; 1 stick cold unsalted butter, cut into small pieces; 1/2 cup plus 2 tablespoons ice water; 1 large egg, beaten; Sea salt flakes, for sprinkling; 1 cup crème fraîche; 1 tablespoon fresh lemon juice; 1/2 teaspoon freshly ground black pepper; 1/2 pound thinly sliced smoked salmon; 1/2 pound smoked mackerel fillets, skinned and broken into pieces; 1/4 pound smoked shrimp; Sliced onions, for serving