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 through a colander.
02 - While the pasta cooks, melt unsalted butter in a large skillet over medium-low heat. Add minced garlic and stir continuously for about 1 minute until fragrant and lightly golden.
03 - Add lemon zest, lemon juice, Dijon mustard, smoked paprika, cayenne pepper, and crushed red pepper flakes if using. Whisk thoroughly until the sauce is smooth and well emulsified.
04 - Stir in chopped parsley, chives, dill, salt, and black pepper. Let the sauce simmer gently for 2 to 3 minutes, allowing the herbs to release their aromatics and the flavors to meld together.
05 - Transfer the drained pasta into the skillet with the cowboy butter sauce. Toss vigorously using tongs or a pasta fork, adding splashes of reserved pasta water as needed to achieve 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.