Step 1

Heat oil and 1 tablespoon butter in heavy skillet over medium-high heat until butter melts and begins to foam. Add mushrooms and a pinch of salt; saute in hot butter and oil golden brown, about 5 minutes. Push mushroom to the edges to clear a space in the center of the skillet.

Step 2

Season halibut on each side with salt and cayenne pepper. Place halibut fillets in the center of the skillet and cook until the fillets begin to flake, 3 to 4 minutes per side. Transfer halibut to a warm serving plate.

Step 3

Pour the water into the pan and bring to a boil while scraping the browned bits of food off of the bottom of the pan with a wooden spoon, until water is reduced by half, 1 to 2 minutes. Pour lemon juice into mushroom mixture and stir.

Step 4

Stir butter and parsley into mushroom mixture. Stir constantly until butter melts and forms a sauce; remove from heat and season with salt and black pepper. Spoon sauce and mushrooms over halibut.