Wilted Spinach Salad with Caramelized Onions & Warm Feta Dressing

• 1/2 lb. spinach
• 5 tablespoons olive oil, divided
• 1 medium red onion, halved, cut into 1/3-inch-thick wedges with some core attached
• 1 7-ounce package feta cheese, coarsely crumbled
• 2 tablespoons Sherry wine vinegar

Place spinach in large bowl. Heat 2 tablespoons oil in heavy large skillet over high heat. Add onion; sauté until brown and softened, about 7 minutes. Transfer to bowl with spinach; remove skillet from heat. Add remaining 3 tablespoons oil and cheese to skillet. Stir to melt cheese slightly, about 1 minute. Stir in vinegar. Season to taste with salt and pepper. Pour over spinach; toss to coat and wilt slightly.