1.Heat the oven to 450F. In an ovenproof pot, heat the oil over moderately high heat. Add the mushrooms and cook until starting to brown, about 3 minutes. Add the scallion bulbs, garlic, and 1/4 teaspoon each of the salt and pepper. Cook until the scallions start to soften, about 4 minutes.

2.Sprinkle the fish with the remaining 1/4 teaspoon each salt and pepper. Put the fish on top of the mushrooms and scallions and transfer the pot to the oven. Roast until the fish is just done, 10 minutes for 3/4-inch-thick fillets. Remove the monkfish from the pot and put the pot on top of the stove. Add the scallion tops and the cream to the pot. Bring to a simmer and continue simmering the sauce until it starts to thicken, 1 to 2 minutes. Serve the fish topped with the sauce.