In a large skillet, heat 2 tsp of oil over medium-high heat. Add fish and cook until opaque in the center, about 2 1/2 minutes per side. Transfer fish to a platter and tent with foil to keep it warm.
Heat remaining 2 tsp 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, caper and crushed red pepper and cook for 3 minutes. Stir in spinach and cook until wilted, about 3 minutes.