In a stockpot, bring water up to a boil. Add spaghetti and cook for 4-5 minutes until al dente. Save a 1/4 cup of the pasta water.
Slice leek greens thinly.
In a frying pan, add butter and olive oil and saute leek greens in melted butter for 2-3 minutes until greens are softened.
Add salt, black pepper, red pepper, and the zest from one lemon and stir.
Add the juice from one lemon and simmer for 2-3 minutes.
Add cooked spaghetti and toss with sauteed leek greens.
Add parmigiano-reggiano and saved pasta water and stir.
Serve as is or with a bread accompaniment. You can keep leftovers for 3-4 days in the refrigerator and reheat in a frying pan.