Combine cheese and butter in a small bowl; stir until well blended.
Grill bread 5 minutes on each side or until golden; cool. Spread 1 teaspoon cheese mixture on each slice; set aside.
Thread 4 fig halves lengthwise onto each of 6 (8-inch) skewers; coat figs with cooking spray. Place fig kebabs on grill rack, and grill 4 minutes on each side. Cool slightly; remove figs from skewers.
Combine vinegar, oil, salt, and pepper in a small bowl. Place spinach and arugula in a large bowl; add dressing, tossing gently to coat. Divide mixture evenly among 8 serving plates. Top with figs and lettuce leaves; sprinkle with onions. Serve with toasts.