In large bowl or on platter, lay arugula. Place cherry tomatoes, cucumber, feta, red onion and dill over arugula. Top with tuna; reserve oil from cans.
To make dressing: In small bowl, combine lemon juice, dill, salt and pepper. Whisk in reserved olive oil from tuna cans until mixture is well blended.
Before serving, drizzle dressing over salad and top with additional chopped dill, if desired.