1. In a large pot of boiling salted water, cook the fusilli until al dente, then drain.
2. Meanwhile, in a large, deep skillet, heat the olive oil. Add the leek and cook over moderate heat until softened, about 10 minutes. Add the cream and simmer over moderate heat until slightly thickened, about 5 minutes. Add the spinach and cook until wilted, about 2 minutes.
3. Add the cooked fusilli to the skillet and toss over moderately low heat until coated with the leek sauce, about 1 minute. Remove from the heat, add the chopped basil and toss. Season with salt and pepper. Spoon the fusilli into bowls and serve.