Preheat oven to 375°F Spray an 8x4 inch loaf pan with cooking spray.
To make meatloaf: In a bowl, mix all meatloaf ingredients. On a 12-inch square piece of waxed paper, pat out mixture into an 8-inch square.
To make filling: In a nonstick skillet sprayed with cooking spray, cook onions over medium heat for 5 minutes until lightly browned. Stir in spinach and garlic; cook for 2 minutes. Turn off heat; add cheeses, stirring until combined. Spread over meatloaf. Using the waxed paper as a guide, roll up the meatloaf; carefully place in loaf pan seam side down. Spread barbecue sauce over top; sprinkle with cheese.
Bake 40 to 45 minutes. Let rest for 10 minutes before carefully inverting onto a serving platter. Slice.