Halve or quarter vine-ripened tomatoes and chop into large pieces until you have 2 cups; place tomatoes in a colander to drain excess liquid for a few minutes.
Drain the tuna cans in a colander or over the sink and set aside.
Peel (optional) and chop cucumbers into large pieces until you have 2 cups.
Coarsely chop the parsley and mint to measure 1 cup each.
Thinly slice green onions (white and green parts) until you have 1/2 cup.
In a small bowl or jar, whisk together olive oil and lemon juice to make a dressing; add any preferred seasoning if desired.
In a large bowl, gently combine the drained tomatoes, cucumbers, green onions, parsley, and mint; toss with the dressing to coat.
Gently fold in the drained tuna pieces, taking care to keep some chunks intact; season with salt and fresh-ground black pepper to taste and serve.