Bring a large pot of salted water to a boil. Cook pasta according to package directions
Melt butter in a medium saucepan over medium heat. Add onion and cook, stirring often, until softened, about 5 minutes. Stir in flour and cook, stirring, for 1 minute. Whisk in clam juice, and then whisk in cream and bring to a boil, whisking continuously.
Drain pasta in a colander and return to pot. Add cream mixture, crabmeat, and salt and pepper to taste and toss well to coat pasta with sauce. Add Parmesan and parsley and toss again. Sprinkle with lemon juice and, if desired, scallion greens.