Arabic Salad or Fattouch Salad

1 lemon
3/4 teaspoon salt
1/4 teaspoon black pepper
3 tablespoons olive oil
2 (1/2-lb) cucumbers, peeled, halved lengthwise, seeded, and cut into 1/4-inch dice (2 1/3 cups)
1 lb tomatoes (3 medium), cut into 1/3-inch dice (2 1/2 cups)
1 cup finely chopped red onion (1 small) or 1 cup chopped scallions (about 5)
1 cup coarsely chopped purslane (optional)
1 cup finely chopped fresh flat-leaf parsley (from 1 large bunch)
1/2 cup finely chopped fresh mint (from 1 bunch)