For an extra touch of elegance, consider adding a small sprig of fresh dill or a thin slice of lemon to each canapé. If you prefer a milder onion flavor, soak the chopped white onion in cold water for 10 minutes, then drain thoroughly before adding it to the relish. The canapés can be assembled up to 2 hours in advance. Cover and refrigerate until ready to serve. However, for the best texture, assemble them closer to serving time.