- 3 grapefruit (preferably 2 white and 1 pink or red)
- 1/4 cup dry white wine
- 1 shallot, minced
- 2 tablespoons white-wine vinegar
- 1 1/4 sticks (1/2 cup plus 2 tablespoons) unsalted butter, cut into tablespoon pieces
- 4 (1/2-inch-thick) halibut steaks with skin (1 1/2 lb)
- 3 tablespoons vegetable oil
- 1 lb fresh shiitake mushrooms, stems discarded and caps thinly sliced
- 2 Belgian endives, trimmed and cut crosswise into 1-inch-thick slices
- Finely grate 1 teaspoon zest from a grapefruit. Squeeze 1/2 cup juice from a white grapefruit.
- Cut peel, including all white pith, from remaining fruit with a sharp paring knife and cut segments free from membranes. Chop enough grapefruit segments (use all colors) to measure 1/2 cup. Reserve 1 cup of remaining whole segments.
- Boil juice, wine, shallot, and vinegar in a small heavy saucepan until reduced to about 1 tablespoon. Reduce heat to low and whisk in butter 1 piece at a time, lifting pan from heat occasionally to cool sauce and adding each new piece of butter before previous one has melted completely (sauce must not get hot enough to separate).
- Stir in chopped grapefruit and season with salt and pepper. Keep beurre blanc warm in a metal bowl set over a saucepan of hot water.
- Preheat oven to 250°F. Pat halibut dry and season with salt and pepper. Heat 1 1/2 tablespoons oil in a large nonstick skillet over moderately high heat until hot but not smoking, then sauté fish, in batches if necessary, turning over once, until golden brown and just cooked through, about 5 minutes total. Transfer fish to a shallow baking pan and keep warm in oven.
- Wipe skillet clean and heat remaining 1 1/2 tablespoons oil over moderately high heat until hot but not smoking, then sauté mushrooms with salt and pepper to taste, stirring, until golden brown, 5 to 7 minutes. Add endives and sauté, stirring, until leaves are slightly wilted, about 1 minute. Stir in reserved whole grapefruit segments and remove from heat.
- Serve fish over mushrooms, topped with beurre blanc and zest.