To prepare the crackers: Combine the flour, pepper, lemon rind, and butter in a large bowl. Blend with fingertips or a pastry blender until the mixture resembles coarse meal. Stir in the sour cream, lemon juice, and 1 tablespoon cold water. Toss to form stiff dough. Turn out onto a floured work surface and knead briefly into a ball. Add a 1 to 2 teaspoons of cold water if the dough appears to be too stiff. Wrap the ball in plastic and refrigerate until well chilled, at least 15 minutes (The dough can be made up to a day in advance).
Preheat the oven to 400 degrees. Cut the ball in half and working with one half at a time, roll the dough out 1/16-inch thick. Use a 2-inch cookie cutter to make 16 rounds. Repeat with the other half of the dough. Transfer to an ungreased cookie sheet and sprinkle with salt. Bake in the middle of the oven for 8 to 10 minutes, or until golden. Cool on wire racks for 5 minutes before serving.
To make the chowder: Peel the potatoes and cut into 1/4-inch cubes. Melt the butter in a saucepan over low heat; add the onion and cook, stirring often until softened, about 5 minutes. Whisk in the flour and cook for about 3 minutes. Increase the heat to medium-high, pour in the hot milk, and bring to a boil, whisking constantly. Add the potatoes, season with salt, and reduce the heat to medium-low. Cook until the potatoes are almost tender, 10 to 12 minutes. Add the salmon, and cook until slightly firm to the touch, about 5 minutes. Transfer the salmon to a plate and break into chunks. Add the chunks, smoked salmon, dill and lemon juice to the hot chowder. Season with salt and pepper and simmer until just heated through, about 2 minutes. Serve hot with crackers on the side.
Recipe courtesy of Sara Moulton, Sara Moulton Cooks at Home, Broadway Books, 2002