In a small bowl, combine the crème fraîche, mustard, dill, and salt. Cover and refrigerate for up to 1 day.Heat broiler on high. Arrange the bread on 2 baking sheets. Broil until lightly toasted, about 1 1/2 minutes per side. Remove from oven. Rub 1 side of each toast with the cut side of a shallot. Cut each piece of toast on the diagonal twice to form 4 triangles. Top each triangle with a slice of salmon and a dollop of the crème fraîche mixture. Serve the lemon wedges on the side.