The sauce gets better the longer it sits, so prepare it up to a day ahead of time.
Cut the cucumber in half lengthwise and scoop out the seeds with a spoon. Cut into thin slices and toss with about 1/2 tsp salt. This will help release excess moisture. Transfer to a colander to let water drip out.
Meanwhile, grate about 1/2 tsp's worth of lemon zest and juice about about 1 tsp. Put those in a mixing bowl and add the dill and yogurt. Stir to combine.
Lightly press the cucumber to release extra water, and shake the colander to get every last bit. Pat the cucumber dry with a paper towel, then add the cucumber to the yogurt. Stir and do a taste test. If it's too thick and not too lemony, add a little more lemon juice. Too thin, add more yogurt. Add salt and pepper if desired.
Before cooking the salmon filets, bring them to room temperature. It takes only about 15 minutes. Pat them dry with a paper towel, then season with salt and pepper.
Heat a skillet over medium-high heat with about 2 tbs olive oil. Working in batches if needed, put the first fillets in skin side up. Cook for about 4 minutes, until the bottom is completely golden brown. Turn the fish over and cook about 2 to 3 minutes more.
Let the filets rest about 3 minutes before adding the sauce. Serve with your favorite sides, and enjoy!