1. Prepare couscous according to package directions.
2. Wipe mushroom caps clean with dampened paper towels. (Remove gills from underside with a spoon, if desired.) Drizzle with 1/4 cup oil, coating both sides, and season with 1 garlic clove, salt, and pepper. Transfer to heated grill pan or outdoor grill, and grill over medium heat 3 to 4 minutes on each side or until cooked. Transfer to a plate, reserving juices.
3. Combine remaining minced garlic and lemon juice in a medium bowl; whisk in remaining 1/2 cup oil. Season with salt and pepper. Fluff couscous with a fork, and transfer to a large bowl. Add tomatoes and next 4 ingredients. Add all but 2 tablespoons dressing to couscous, and toss to combine.
4. Toss mixed greens with remaining 2 tablespoons dressing in a separate large bowl; arrange greens around rim of 6 serving plates. Mound couscous mixture in center. Slice mushrooms diagonally, and arrange on top. Pour mushroom juices over salad, and garnish, if desired.