Cook the couscous according to package directions. 1 cup dry makes 1 1/2 cups cooked couscous. After the couscous is cooked, drain using a fine mesh strainer, and then rinse it immediately with cold water, and then drain well. The couscous will seem sticky straight from the pot, but no worries, it won't be sticky after it's rinsed and drained.
In a large bowl, add 1/2 cup olive oil, chopped onion, and 2 tablespoons lemon juice, whisk until combined. Stir in the couscous, cucumber, tomatoes, parsley and mint. Add in a bit of salt and pepper to taste and toss to combine.
Serve this salad right away or store covered in the refrigerator for later. If you prepare this salad the day before for an event, just omit the mint and parsley, cover and chill the salad, and then stir in the freshly chopped mint and parsley just before serving.