Step 1Heat 4 tablespoons butter in a large cast-iron skillet over medium-high heat. Add potatoes to skillet and cook for 8 minutes, stirring only once or twice, until browned and beginning to crisp.
Step 2Add the remaining tablespoon butter to the skillet along with the onions. Cook the onions, stirring occasionally, until they have softened, about 10 minutes. Add the salmon, cream, horseradish, mustard, garlic and dill to the skillet, as well as salt and pepper to taste. Stir gently to combine.
Step 3Continue cooking the mixture over medium-high heat for another 10 minutes, turning the hash in parts every few minutes and loosening any crusty bits, until potatoes are well browned. Stir in the capers.
Step 4In a small bowl, combine creme fraiche and lemon juice to taste. Remove the hash from heat. Serve with a spoonful of creme fraiche and garnished with chopped chives.