In a large nonstick skillet, heat 1/2 tablespoon of the olive oil until almost smoking. Season the monkfish with salt and white pepper and add 8 of the medallions to the skillet. Cook over moderately high heat until nicely browned on the bottom, about 3 minutes. Turn the medallions and cook until just opaque throughout, about 1 minute. Transfer the medallions to a warm platter, cover with foil and repeat with the remaining monkfish and oil. Wipe out the skillet and add the cooked spinach. Cook over high heat, stirring, until warmed through. Season with salt and white pepper.