- 1 lb portobello mushrooms, stems discarded and caps sliced 1/2 inch thick
- 1/4 cup extra-virgin olive oil
- 1 cup reduced-sodium chicken broth
- 1 tablespoon Sherry vinegar or rice vinegar (not seasoned)
- 1 tablespoon balsamic vinegar
- 1 tablespoon minced shallot
- 1/4 teaspoon minced garlic
- 3/4 teaspoon kosher salt
- 2 cups water
- 1 1/2 lb baby spinach
- 2 tablespoons unsalted butter
- 4 (1 1/4-inch-thick) pieces halibut fillet (1 1/2 lb total), skinned
- 2 tablespoons extra-virgin olive oil
- 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.
- 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.
- 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.
- Pat halibut dry and season with salt and pepper.
- 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.
- Arrange halibut, spinach, and mushrooms on 4 large plates or bowls. Spoon vinaigrette over each serving.