In a large skillet, heat 1 tablespoon oil over medium-low heat. Add leeks, 3/4 teaspoon salt, and 1/4 teaspoon pepper. Cook, stirring often, until softened, about 10 minutes.
Add beans, broth, and sage to pan; with a potato masher, mash about one third of beans. Bring to a boil; reduce to a simmer. Cook until sauce has thickened but is still soupy, about 10 minutes. Cool; transfer to a covered container, and refrigerate until ready to use, up to 2 days.
Meanwhile, in a large pot of boiling salted water, cook pasta 2 minutes less than package instructions suggest. Drain; rinse under cold water until cool.Transfer to a bowl, and toss with remaining tablespoon oil to coat. Cover and refrigerate until ready to use, up to overnight.
In a large (5-quart) saucepan over medium heat, bring sauce and lemon juice to a simmer. Stir in cream and Parmesan. Add pasta; cook, stirring, until heated through, 2 minutes. Serve sprinkled with parsley and more cheese, if desired.