01 - Bring a large pot of salted water to a rolling boil. Cook fettuccine or spaghetti according to package directions until al dente. Reserve 1/2 cup of starchy pasta water before draining in a colander.
02 - While the pasta cooks, melt unsalted butter in a large skillet over medium-low heat. Add minced garlic and stir gently for about 1 minute until fragrant and softened, being careful not to brown it.
03 - Add lemon zest, fresh lemon juice, Dijon mustard, smoked paprika, cayenne pepper, and crushed red pepper flakes if using. Whisk everything together until the sauce is smooth and well blended.
04 - Stir in chopped parsley, chives, dill, salt, and black pepper. Let the sauce simmer gently for 2 to 3 minutes so the herbs release their aroma and the flavors meld together.
05 - Transfer the drained pasta directly into the skillet with the cowboy butter sauce. Toss vigorously using tongs or a pasta fork, adding reserved pasta water a splash at a time until the sauce clings to every strand in a silky, glossy coating.
06 - If using grilled steak, chicken, or shrimp, fold it into the pasta now and toss until heated through, about 1 to 2 minutes.
07 - Serve immediately in warm bowls, garnished with extra fresh herbs and freshly grated Parmesan cheese if desired.