In a large nonstick skillet, heat 2 teaspoons of oil over a medium-high heat. Add fish and cook until opaque in the center, about 2 1/2 minutes per side. Transfer the fish to a platter and tent with foil to keep the fish warm.
Heat the remaining 2 teaspoons of oil in the same skillet; add onion and saute for 2 minutes. Add the wine and cook until reduced by half, about 2 minutes. Add the tomatoes, olives and capers and crushed red pepper, if using, and cook for 3 minutes more. Stir in the spinach and cook until it is wilted, about 3 minutes. Season with salt and pepper. Spoon the sauce over the fish and serve.