How to Make It
In a large skillet, melt 2 tablespoons of the butter, then cook over moderately high heat until lightly browned, about 1 minute. Add the capers and cook until the skins burst, about 2 minutes. Transfer to a paper towel–lined plate.
Spread the flour in a wide, shallow dish. Season the sea bass with salt and black pepper and dredge in the flour; tap off the excess.
Melt the remaining 4 tablespoons of butter in the skillet. Add the sea bass skin side down and cook over moderately high heat until the skin is crisp and golden, about 5 minutes. Flip the fish and cook until white throughout, about 4 minutes; transfer to plates. Add the leek, parsley and lemon zest to the skillet and cook, stirring frequently, until the leek is tender, about 2 minutes. Add the verjus and simmer until reduced by half, about 3 minutes. Stir in the yuzu juice and fried capers, then spoon the sauce over the fish; keep warm.
In another large skillet, heat the olive oil until shimmering. Add the garlic and crushed red pepper and cook over moderately high heat until the garlic is softened, about 1 minute. Add the arugula and cook, stirring frequently, until wilted, 1 to 2 minutes; season with salt and black pepper. Serve with the fish
Sweet, slightly acidic verjus is the pressed, unfermented juice of unripe grapes. It is available at specialty food stores. Yuzu juice, from the Japanese citrus fruit of the same name, is available at Japanese markets and online at marxfoods.com.You have to pay attention when cooking flounder. If the pan is too hot, the fish turns to mush; too cool, and it goes rubbery.