In a medium pan over medium-high heat, saute onion in oil until softened and slightly caramelized, about 10 minutes. Season with salt and pepper. Add vinaigrette and cranberries. Place spinach in a salad bowl, pour warmed vinaigrette mixture over salad and toss lightly. Divide among four plates and sprinkle with pumpkin seeds. Serve immediately.
Warm Cranberry Vinaigrette
In a small bowl, combine vinegar, mustard, mustard powder, salt, pepper, garlic, juice, and honey. Add walnut oil in a slow stream, whisking constantly. Yield: about 1 cup