1 medium onion, finely chopped (about 1 cup)
1 tablespoon olive oil
3 cups reduced-sodium chicken broth (24 fl oz)
1 cup heavy cream
1/3 cup vodka
1/2 teaspoon salt
1/2 cup chopped fresh dill
1 1/2 teaspoons finely grated fresh lemon zest
2 tablespoons fresh lemon juice
1/4 teaspoon coarsely ground black pepper
2 cups flaked broiled salmon
10 oz capellini (angel-hair pasta; about two thirds of a 1-lb box)
Cook onion in oil in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until softened (but not browned), about 6 minutes. Add broth, cream, vodka, and salt and boil over moderately high heat, stirring occasionally, until sauce is reduced to 2 cups, 40 to 50 minutes. Remove from heat and stir in dill, lemon zest and juice, and pepper.
Reserve 1/2 cup sauce, then add salmon to saucepan and cook over moderately low heat until fish is just heated through, 2 to 3 minutes. While fish is heating, cook pasta in a 6- to 8-quart pot of boiling salted water until al dente. Reserve 1/2 cup pasta-cooking water, then drain pasta in a colander.
Return pasta to pot, then toss with reserved sauce and cooking water. Serve pasta immediately with fish and sauce spooned over the top.