1. Preheat broiler to high, line a broiling pan with foil and set rack 2 inches from heat. In a small bowl, combine 3 Tbsp. olive oil, lemon juice and garlic; season with salt and pepper.
2. Warm remaining oil in a skillet over medium-high heat. Sauté onion until softened, 3 minutes. Add frozen spinach, cover and cook until thawed, about 8 minutes. Uncover, season with salt and pepper and drizzle with 1/2 of flavored olive oil. Cook for 4 minutes to simmer off excess liquid. Remove from heat, cover and keep warm.
3. Lightly mist foil-lined broiling pan with cooking spray. Arrange flounder fillets on top and season with salt and pepper. Drizzle remaining flavored oil over fish and broil, undisturbed, until opaque and lightly browned, about 6 minutes.
4. Divide spinach among 4 plates. Place fish on spinach bed and serve hot, topped with lemon zest, if desired.