Boil salted water and cook pasta until al dente, reserving 1 cup of pasta water.
Toast breadcrumbs with lemon zest in some butter until golden, then set aside.
Melt butter in a large pan over medium heat. Add sliced leeks, season with salt and pepper, and cook until soft and jammy, about 5–7 mins.
Pour in heavy cream and let simmer for a few minutes until slightly thickened. Stir in lemon zest.
Add cooked pasta and parmesan cheese, tossing to coat. Reduce heat and add pasta water as needed until glossy and saucy.
Add fresh lemon juice, to taste. Taste and adjust salt and pepper as needed.
Serve and top with extra parmesan and toasted breadcrumbs.
