Place red onion in a small bowl, and cover with water. Let soak 30 minutes; transfer to paper towels, and drain.
Lightly coat a medium saucepan with cooking spray, and place over medium heat. Add couscous; cook, stirring constantly, 1 minute. Add the water, and bring to a boil. Add 1 teaspoon salt; cook until all water has been absorbed and couscous is al dente, about 8 minutes. Remove from heat; let cool completely.
In a large bowl, combine couscous, reserved red onion, parsley, mint, oil, and lemon juice; toss to combine. Season with remaining 1/2 teaspoon salt. Serve, garnished with lemon wedges.