Step 1

In a small bowl, combine the onion with 2 tablespoons of the vinegar and let sit for at least 10 and up to 30 minutes; this will mellow the onion's bite.

Step 2

Meanwhile, in a small bowl, whisk together the oil, lemon juice and the remaining 2 tablespoons of vinegar until combined. Season to taste with salt and pepper. Taste, and, if desired, adjust the amounts of vinegar, juice and/or oil.

Step 3

Drain the onions; discarding vinegar or reserving it for another use. In a large, shallow bowl, gently toss together the onion, tomatoes, cucumber, bell pepper and parsley. Drizzle with as much of the vinaigrette as desired, and toss gently to coat. Serve immediately or let rest at room temperature for up to 30 minutes.