Dice cucumber into 1-inch rounds.
Chop red onion and olives into small pieces.
Break tuna into large chunks and mix with 1 tbsp olive oil and 1 tsp lemon juice.
In a bowl, combine cucumber, cherry tomatoes, and half the diced red onion. Add 1 tbsp olive oil and toss gently.
In another bowl, mix feta cheese with olives, remaining cherry tomatoes, and remaining red onion. Add salt and pepper.
Layer jars: start with the cucumber-vegetable base, add a few tuna pieces, top with feta mixture. Drizzle with remaining olive oil and tuck dill in the center.
Refrigerate 30 minutes before serving for optimal flavor fusion.