Wash arugula thoroughly and pat dry. Halve cherry tomatoes and chop cucumber into bite-sized pieces. Thinly slice red onion.

In a large bowl, combine arugula, cherry tomatoes, cucumber, and red onion. Toss gently.

Flake the drained tuna over the vegetable mixture and mix carefully.

Slice the avocado in half, remove the pit, scoop out the flesh, cut into cubes, and add to the salad.

Whisk together lemon juice and olive oil in a small bowl; drizzle over salad and toss gently to coat.

Serve immediately or chill for a refreshing meal.