Open and drain the tuna. Flake it into a large mixing bowl with a fork so the pieces are bite-sized.

Add the cooked and fully cooled pasta to the tuna. Gently fold so the tuna coats the pasta without turning it mushy.

Toss in the halved cherry tomatoes, diced bell pepper, diced cucumber, and finely chopped red onion. Mix briefly to combine.

In a small bowl, whisk the mayonnaise with the lemon juice, a pinch of salt, and a few grinds of black pepper. Taste and adjust the lemon or salt as needed.

Pour the dressing over the pasta and vegetables. Toss gently but thoroughly until everything is lightly coated. Do not overmix or the tuna will break down too much.

Cover and chill the salad in the refrigerator for at least 30 minutes to let the flavors meld.