Step 1: Cook pasta according to package directions until firm to the bite. Drain, reserving 1 cup of cooking liquid.
Step 2: In a large skillet over medium-high heat, cook olive oil and butter until sizzling. Add garlic and red pepper flakes; cook and stir 30 seconds. Add shrimp; cook and stir until pink, 3 to 4 minutes. Remove from heat.
Step 3: Once pasta is cooked and drained, return shrimp mixture in skillet to medium heat. Add pasta and ¼ cup of cooking liquid; stir until heated through, adding more cooking liquid if pasta is too dry.
Step 4: Remove skillet from heat and stir in parsley and lemon juice and zest. Season to taste with salt and pepper. Transfer to serving platter or bowl and sprinkle with parmesan cheese if desired. Serve.