Smear the softened butter on the bread. Dollop the mustard on top of the butter and smear in an even layer. Top with the thinly sliced cucumber, overlapping slightly. Season with salt and pepper.
Arrange the smoked salmon on top in folded ribbons. Garnish with red onion and fresh dill and sprinkle with flaky sea salt and more black pepper. Serve with a lemon wedge on the side. Enjoy!
