Bring a large pot of salted water to a boil. Cook the spaghetti or linguine until al dente according to the package instructions.
Reserve ½ cup of pasta water before draining.
Make the lemon pepper mixture
Heat the butter and olive oil in a large skillet over medium heat. Add the garlic and cook for 30 seconds, until fragrant.
Stir in the black pepper, lemon zest, lemon juice, and optional red pepper flakes. Cook for 1 minute.
Toss the pasta
Add the cooked pasta and toss to coat. Reduce the heat to low. Add the reserved pasta water or whole milk a little at a time while tossing.
Remove the skillet from the heat. Gradually sprinkle in the Parmesan, tossing until the sauce is smooth and coats the pasta.
Finish and serve
Taste and add kosher salt as needed. Sprinkle with the chopped parsley and serve warm.
Notes
Parmesan: Remove the skillet from the heat before adding freshly grated Parmesan. Sprinkle it in gradually while tossing to prevent clumping.
Adjust the heat: For mild pasta, use 1 teaspoon black pepper. For medium heat, use 2 teaspoons. For extra heat, use 1 tablespoon black pepper plus ½ teaspoon red pepper flakes.
Substitutions: Use whole milk instead of pasta water for a creamier sauce. Substitute 1 teaspoon lemon pepper seasoning for the black pepper and zest. Keep the lemon juice, then adjust the seasoning and salt to taste.
Storage and reheating: Refrigerate leftovers in an airtight container for up to 3 days. Reheat gently over low heat with a splash of water or milk.