Combine the creme fraîche, mustard, dill, and salt. Cover and refrigerate up to 1 day ahead.
Heat broiler. Arrange the bread slices on baking sheets. Toast until lightly browned; turn and toast the other sides. Remove from oven and rub 1 side of the toasts with a cut shallot. Cut each slice into 4 triangles. Serve with the salmon, sauce, toasts, and lemon slices.
Note: Salmon is expensive, but it's elegant when served on a big platter and guests make their own open-face sandwiches. Mix leftover salmon and sauce with cooked spaghetti the next day.