Preheat oven to 450°. Line a rimmed baking sheet with aluminum foil.
In a small bowl, combine red onion, olive oil, salt, and pepper; toss to coat. Place on prepared pan. Bake for 15 to 20 minutes, or until lightly browned around edges. Set aside to cool.
In a medium bowl, combine mesclun greens, arugula, and fennel. Divide mesclun mixture evenly among 4 salad plates. Top each salad with roasted red onion and figs. Garnish with cheese and cashews, if desired. Drizzle with Molasses Vinaigrette just before serving.