Ludo Lefebvre’s tutorial on how to build a deeply layered, silky sauce: reduce, one at a time, wine, citrus juice, and crème fraîche, then blend in fresh citrus segments and curry powder for a jolt of vibrant flavor. Slideshow: More Salmon Recipes
Using a sharp knife, peel the orange and the grapefruit, removing all of the bitter white pith, then cut in between the membranes to release the sections.
In a small saucepan, simmer the wine with the vermouth and shallots over high heat until almost evaporated. Add the orange and grapefruit juices, bring to a simmer and cook until syrupy. Add the crème fraîche and bring to a simmer. Reduce the heat to low and cook, stirring occasionally, for 30 minutes. Remove the pan from the heat and strain the sauce through a fine sieve into another small saucepan. Add the citrus sections and curry powder and, using an immersion blender, puree the sauce until smooth. Season with salt and pepper.
In a large nonstick skillet, melt 2 tablespoons of the butter. Season the salmon on both sides with salt and pepper. Cook the salmon skin side down over medium-high heat until browned and crisp on the bottom, about 3 minutes. Turn and cook until almost opaque through, about 3 minutes. Transfer the salmon to a platter and tent with foil.
In the same skillet, melt the remaining 2 tablespoons of butter. Add the amaranth and cook over moderately high heat, stirring occasionally, until just wilted. Mound the amaranth on plates and top with a salmon fillet. Spoon some of the sauce over the top and serve. Pass the remaining sauce separately.