Cold-smoked salmon already brings enough salt and richness that a thin layer of lemon-dill butter is all it needs on soft rye. Lay the slices flat rather than folding them into high ruffles, because narrow tea fingers cut much more cleanly through one even fish layer. The butter should be soft enough to spread without tearing the bread but not shiny and melting at the edges. Chill the closed sandwiches briefly before trimming, then keep them covered so the exposed salmon stays moist. There’s no reason to crowd these with cucumber and onion when those fillings already have plenty of room elsewhere on a party tray.