To a pot of salted water, boil spaghetti for 6 minutes.
To a skillet, add in butter and oil. Allow to melt and add in lemon juice and zest. Follow with black pepper and 1 cup of reserved pasta water. Stir to combine and make a sauce.
Add in undercooked spaghetti and stir frequently to allow the starches to release into the skillet. This will create a silky sauce.
Once pasta is at an at dente texture remove from heat. Serve immediately and top with parsley and parmesan.