Soften the celery in 1 tbsp oil and the butter for 5 minutes in a large pan, then add the leeks and soften for 5 more minutes. Stir in the garlic, cook for a minute, then add the stock and bring to the boil. Tip in the pasta, boil for 4 minutes then add the greens and cook for another 4 minutes until the pasta and the greens are tender. Stir in the chopped basil. Stir in the lemon juice and zest and divide between bowls. Scatter with a little cheese to serve.