Bring a pot of water to a boil and cook spaghetti according to package instructions.

While spaghetti is cooking heat olive oil in a pan over medium heat and add onion and garlic. Sauté until onion starts to become tender - approx. 5 minutes. Add red pepper flakes, pepper, lemon juice and lemon zest to the pan and cook for an additional 2 minutes.

Add drained pasta, butter and parmesan to the pan and stir to combine. Plate pasta and top with fresh parsley and red pepper flakes.