Simple Dinners: Double Cheddar Stuffed BBQ Turkey Burgers

For the onions:

• 1 large red onion
• 2 tablespoons olive oil
• 1 teaspoon salt
• 1 teaspoon sugar

For the turkey burgers:

• 1 pound lean ground turkey
• 1/2 cup baby spinach leaves, finely chopped
• 1 large egg
• 1 tablespoon Kraft Brand BBQ sauce plus more for drizzling
• 1 teaspoon salt
• 1/2 teaspoon ground black pepper
• 4 slices Cheddar Cheese, each slice cut into 4 (like a square)
• 4 slices Kraft VELVEETA cheese slices, each slice cut into 4 (like a square)
• 4 hamburger buns