In a large pot of salted boiling water, cook pasta according to package directions until al dente. Drain, reserving ½ cup pasta water, and return to pot. In a large skillet over medium heat, heat oil. Add onion and cook until soft, 5 minutes, then add garlic and cook until fragrant, 1 minute more. Add wine and cook until almost completely reduced, 5 minutes. Add heavy cream and lemon juice and cook until thickened, another 5 minutes. Season with salt and pepper.  Add salmon, capers, and dill and cook until salmon is warmed through, 2 minutes, then toss sauce and pasta together. Add ¼ cup pasta water if sauce is too thick. Garnish with more dill to serve.