In a deep 10-inch skillet, heat 2 tablespoons of the oil. Add the fennel and season lightly with salt and pepper. Cover and cook over low heat, stirring, until lightly browned and tender, 20 minutes. Add the scallions, cover and cook until they are just tender, 2 minutes. Transfer to a bowl. Add the remaining 1 tablespoon of oil to the skillet and add the curry powder. Cook over low heat, stirring, until fragrant, about 1 minute. Add the wine and boil over high heat until reduced by half, about 1 minute. Add the clam juice and boil until reduced by half, about 3 minutes. Cover and remove the skillet from the heat.
Preheat the broiler. Arrange the salmon steaks on a large rimmed baking sheet and season with salt and pepper. Broil about 6 inches from the heat for 3 to 4 minutes, until just opaque throughout.
Meanwhile, scrape the fennel and scallions into the sauce and reheat; stir in the lemon juice. Remove the skillet from the heat and stir in the butter, 1 tablespoon at a time, until incorporated. Season with salt and pepper.
Transfer the salmon to plates and spoon the sauce on top. Garnish the salmon with fennel fronds and serve.