Bring a large pot of salted water to boil. Cook pasta in boiling water until firm to the bite (al dente), about 8 minutes. Drain, reserving 1 cup of pasta water.

Meanwhile, heat olive oil and butter in a large skillet over medium-high heat until sizzling. Add minced garlic and red pepper flakes; cook and stir until fragrant, about 30 seconds. Add shrimp; cook and stir until shrimp is just cooked through, 3 to 4 minutes. Remove from heat.

Once you've drained the pasta, set skillet with the shrimp mixture over medium heat. Add drained, cooked pasta and ¼ cup of reserved pasta water; toss to coat. Cook and stir until heated through, adding more pasta water if pasta seems too dry.

Remove pasta from heat and stir in fresh lemon juice and zest, and parsley. Season to taste with salt and pepper. Transfer to serving bowl or platter; top with grated parmesan cheese if using and serve.