Vegetables:
6 tablespoons unsalted butter
1 small leek, white part only, halved and sliced thin crosswise, washed
6 shiitake mushrooms, stemmed and sliced thin
1/4 cup chicken stock
1/4 cup hazelnuts, toasted, peeled and chopped coarse
1/4 pound Brussels sprouts, tough outer leaves discarded, remaining leaves separated
Kosher salt and freshly ground pepper

Fish and sauce:
2 tablespoons olive oil
4 (5-ounce) sea bass fillets
Kosher salt and freshly ground pepper
3 tablespoons unsalted butter
1/2 cup ice-cold chicken stock
Lemon juice
1 teaspoon chopped flat-leaf parsley

Make the vegetables: Melt 2 tablespoons of the butter in a large non-stick skillet over medium heat. Add the leeks and cook for about 3 minutes until tender, without browning. Transfer to a bowl. Return the pan to the stove and melt 2 tablespoons of the butter over high heat. Add the shiitakes and cook for 3 minutes until lightly browned on the edges. Transfer to the bowl with the leeks.

Make the fish: Heat a skillet over high heat. Add the olive oil and heat for 1 minute. Season the fish with salt and pepper and add to the pan just as the oil starts to smoke. Cook for 3 to 4 minutes per side, depending on thickness, until firm.

While the fish is cooking, finish the vegetables. Add the stock to the pan and heat over medium-high heat. Whisk in the butter. Add the leeks, mushrooms and hazelnuts. Add the Brussels sprouts leaves and cook until slightly wilted. Season to taste with salt and pepper. Divide among four serving plates.

Place a piece of fish on each plate. Pour out any oil that remains in the pan and return the pan to high heat. Add the butter and cook for 2 to 3 minutes until it begins to turn brown. Add the stock and lemon juice. Cook for 1 to 2 minutes or until slightly reduced. Season to taste with salt and pepper (if the lemon is too tart, add a pinch of sugar to bring into balance). Pour a small amount around the edge of each plate and over the fish. Serve immediately.