Bring a large pot of salted water to a rolling boil over high heat. Add the dried spaghetti and cook until al dente, approximately 8-10 minutes. Reserve 1 cup of pasta cooking liquid, then drain and set aside.

In a large skillet, heat olive oil and butter over medium-high heat. Add minced garlic and crushed red pepper flakes, stirring for about 30 seconds until fragrant.

Add the medium shrimp to the skillet, cooking for 3-4 minutes until they are pink and opaque. Remove from heat.

Return skillet to medium heat, add the drained spaghetti and 1/4 cup of the reserved cooking liquid, tossing for 1-2 minutes to combine.

Remove from heat and stir in lemon juice, lemon zest, and parsley. Season with salt and pepper, and add Parmesan cheese if desired before serving.