Peel and dice onions. Rinse basil, shake dry, pluck leaves. Set aside a few leaves for garnish. Cook pasta.
Meanwhile, heat butter in a saucepan, sauté onions. Add broth, bring to a boil and simmer for 2 min. Add crème fraiche, cream and basil, bring to the boil and blend with a hand blender until frothy.
Season sauce with salt and pepper and grated Parmesan.
Drain the pasta. Reserve some of the pasta water. Mix pasta with sauce and add some cooking water. Let stand for a few minutes.
Garnish pasta with basil and serve with grated Parmesan.