Seared Halibut with Portobello Mushroom Vinaigrette

Prepare mushrooms and vinaigrette:

Step 1

Cook mushrooms in 1 tablespoon oil in a 12-inch heavy skillet over moderately high heat, stirring occasionally, until softened, about 5 minutes. Add broth and simmer until mushrooms are tender, about 5 minutes.

Step 2

Transfer mushrooms with a slotted spoon to a plate and keep warm, covered. Boil liquid in skillet until reduced to about 1/3 cup. Transfer to a bowl and cool. Whisk in remaining oil, remaining vinaigrette ingredients, and pepper to taste.

Steam spinach:

Step 3

Bring water to a boil in a large saucepan. Add half of spinach and cook, covered, until leaves are wilted, 1 to 2 minutes. Stir in remaining spinach and cook, covered, until tender, about 2 minutes more. Drain well in a colander, pressing on spinach with a large spoon. Return spinach to pan and stir in butter and salt and pepper to taste. Keep warm, covered.

Prepare halibut:

Step 4

Pat halibut dry and season with salt and pepper.

Step 5

Heat oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then cook fish until golden, about 3 to 4 minutes on each side.

Step 6

Arrange halibut, spinach, and mushrooms on 4 large plates or bowls. Spoon vinaigrette over each serving.