If necessary, boil the eggs for 9–10 minutes, cool them in cold water, peel, and chop.
Drain the tuna thoroughly and break it into medium flakes.
Wash, dry, and thinly slice the cucumbers. Finely chop the red onion and dill.
Mix the yogurt, mustard, salt, black pepper, and optional lemon juice in a large bowl.
Add the tuna and stir until coated with the dressing.
Add the cucumber, onion, and dill. Mix gently.
Fold in the chopped eggs last.
Taste, adjust the seasoning, and serve immediately or chill for 10–15 minutes.
Notes
Use thick, unsweetened yogurt for the creamiest dressing. Drain the tuna well and dry the cucumber after slicing to prevent excess liquid. Add the eggs last so they retain their shape.