Make the sauce by bringing the white wine, orange juice, saffron, sugar and shallot to a boil in a small pot. Simmer strongly for 5 minutes, then turn off the heat and puree the sauce in a blender. Return it to the pot and turn the heat to low. Add salt to taste and keep warm, but don’t boil it.
Cook the greens in 2 tablespoons of butter or oil over high heat in a large saute pan, stirring constantly until they wilt. Add the splash of water, the orange zest and some salt and cover the pot. Lower the heat to medium-low and steam the greens for 2 minutes. Turn off the heat.
Sear the fish by heating the remaining butter in a pan large enough to hold the fish — if you don’t have such a pan, put a baking sheet in the oven and set the oven to 200°F so you can keep the fish warm as you do this in batches. Heat the butter over high heat until it stops frothing. Pat the fish dry with paper towels and set it skin-side down in the hot butter. Turn the heat down to medium-high for a typical trout fillet (like you see in the picture) or to medium if you are working with a thicker piece of fish.
Let the fish cook undisturbed for 2 minutes, then use a large spoon to baste the meat side of the fish with the hot butter. Do this constantly for 90 seconds, then give it a rest. Thin pieces of trout will only need this quick basting to cook through, but thicker pieces of fish will need a second or even third round of basting. When you are done basting, salt the meat side. The skin side should lift off the pan easily after about 4 to 5 minutes of steady cooking. The moment you take the fish off the heat, salt the skin side.
To serve, swirl in the remaining 2 tablespoons of butter into the sauce, one tablespoon at a time. Pour some sauce on everyone’s plate. Top with the greens and then with a piece of fish. Serve at once.