Cook the fusilli in a pot of boiling salted water till done, then drain.
Separate the spinach stems from the leaves, and chop/slice both.
Meanwhile, heat the olive oil in a large pan over a moderate heat.
Add the chopped onion/leek and cook for 10 minutes.
Add the spinach stems and garlic and cook for 5 minutes.
Add the cream, basil, lemon zest and spinach leaves and cook for 2-3 minutes.
Add the cooked fusilli to the creamy mixture and toss it all together.
Season well and serve.