Place tomatoes in a shallow baking dish coated with cooking spray. Bake at 425° for 30 minutes. Set aside.
Heat a medium saucepan over medium-high heat. Add 1 cup diced onion; sauté 3 minutes. Stir in tomatoes, red wine, oregano, pepper, and 1/4 teaspoon salt; bring to a boil. Reduce heat; simmer 20 minutes. Place tomato mixture in a blender; process until smooth. Set aside; keep warm.
Place a saucepan coated with cooking spray over high heat. Add sliced onion; sauté 5 minutes. Add wine and garlic. Bring to a boil; cook 5 minutes. Stir in broth; bring to a boil. Reduce heat; simmer 20 minutes. Place garlic mixture in a blender; process until smooth. Set aside; keep warm.
Sprinkle 1/4 teaspoon salt over eggplant. Place half of eggplant in a single layer on a baking sheet coated with cooking spray; broil 5 minutes on each side or until lightly browned. Repeat procedure with remaining eggplant; set aside.
Place a large nonstick skillet coated with cooking spray over medium-high heat. Add 1/2 cup diced onion; sauté 3 minutes. Add spinach; cook 10 minutes, stirring frequently. Remove from heat; stir in cheese.
Preheat oven to 425°.
Arrange 6 eggplant slices, 2 to 3 inches apart, on a baking sheet. Spread 2 1/2 tablespoons spinach mixture over each slice. Stack each with another eggplant slice, an additional 2 1/2 tablespoons spinach mixture, and remaining slices. Bake at 425° for 15 minutes. Arrange 1 eggplant stack on each of 6 plates; spoon 1/3 cup tomato sauce and 2 tablespoons garlic sauce on each plate. Garnish with oregano, if desired.