How to Make It
In a small saucepan, cook 2 tablespoons of the butter over moderate heat until browned and nutty-smelling, about 6 minutes. Add the shallot and cook, stirring, for 1 minute. Stir in the stock, lemon juice, capers and Worcestershire. Stir in 5 tablespoons of the butter, 1 tablespoon at a time, until incorporated. Stir in the chives. Remove the sauce from the heat and keep warm.
In a very large skillet, heat 1/4 cup of the olive oil over moderately high heat until shimmering. Season the fish with salt and cook until browned on the bottom, about 5 minutes. Flip the fish and cook, basting with the oil in the skillet, until a small knife inserted in the center of the fish near the bone feels warm to the touch and the flesh is opaque, about 3 minutes. Add the remaining 1 tablespoon of butter to the skillet along with the chopped garlic and thyme and continue basting for 30 seconds. Carefully transfer the fish to plates.
Wipe out the skillet and heat the remaining 1 tablespoon of oil over high heat. Add the spinach and cook, stirring, until wilted, about 30 seconds. Stir in the sliced garlic and season with salt. Spoon the spinach alongside the fish and serve with the sauce.