Sprinkle Southwest Seasoning on tuna. Heat oil in a large nonstick skillet over medium-high heat. Add tuna; cook 3 to 5 minutes on each side or until fish flakes with a fork.
Toss greens with desired amount of Cilantro-Lime Vinaigrette. Arrange greens on individual serving plates. Top with tuna, and drizzle with remaining vinaigrette.