Meanwhile, heat 1 tablespoon oil in a large skillet over medium heat. Add fennel and shallot and cook, stirring frequently, until softened and starting to brown, about 5 minutes. Add flour and stir to coat. Add broth and wine and bring to a simmer. Cook, stirring occasionally, until slightly thickened, about 2 minutes. Remove from heat. Stir in crème fraîche, 1/4 cup chives, lemon juice, the fennel fronds and the remaining 1/2 teaspoon salt. Transfer to the bowl with the pasta.