How to Make It
Preheat the oven to 450°. In a skillet, melt 4 tablespoons of the butter. Add the scallions and cook over low heat until softened, about 3 minutes. Add the asparagus spears and the water and cook until tender and the water is reduced to 1/4 cup, about 5 minutes. Add the spinach and cook just until wilted, about 2 minutes. Puree the mixture in a blender until smooth; season with salt and pepper. Transfer the sauce to a small saucepan and keep warm.
Arrange the prosciutto slices in pairs, overlapping them slightly. Place a halibut fillet in the center of the prosciutto and top each fillet with 2 sage leaves. Season with salt and pepper and wrap the prosciutto around the fish.
In a large ovenproof skillet, melt the remaining 1 tablespoon of butter in the olive oil. Add the halibut and cook over high heat until browned on the bottom, about 3 minutes. Carefully flip the fish and add the asparagus tips to the skillet. Transfer to the oven and roast the prosciutto-wrapped halibut for 5 minutes.
Spoon the asparagus-spinach puree onto plates. Arrange the prosciutto-wrapped halibut and asparagus tips alongside and serve immediately.
A minerally Rhône white.