Place the eggs in a saucepan, cover with cold water, and bring to a boil. Cook for 9–10 minutes, then transfer to cold water and cool for 10 minutes. Peel and chop.
Slice or dice the cucumbers. Sprinkle with 1 teaspoon salt and let them sit for 10 minutes. Drain the released liquid well.
Thinly slice the onion. For a milder flavor, soak it in cold water for 5 minutes, then drain.
Chop the fresh dill finely. Drain the canned tuna and flake it with a fork.
In a small bowl, mix Greek yogurt or sour cream with Dijon mustard and black pepper.
In a large bowl, combine cucumbers, onion, dill, tuna, and eggs. Add the dressing and mix gently.
Chill for 15–20 minutes before serving for the best flavor.
Notes
For a lighter version, use Greek yogurt and tuna packed in water. For a richer version, use sour cream and tuna packed in oil. Drain cucumbers and tuna very well to keep the salad creamy, not watery.