Bring a large pot of heavily salted water to a boil. Cook the pasta according to package directions until al dente. Reserve 1 cup of the starchy pasta water before draining. Set aside.

While the pasta cooks, make the green sauce. Add the basil, spinach, queso fresco, toasted walnuts, garlic, and milk to a blender and blend on high until completely smooth and vibrantly green. With the blender running, stream in the olive oil slowly until fully emulsified and silky. Season with salt and black pepper. Taste and adjust seasoning as needed.

If making the steak, season the steaks generously on both sides with salt and black pepper and let sit at room temperature for 10 minutes.

Heat the neutral oil in a large cast iron or heavy-bottomed skillet over high heat until screaming hot. Add the steaks and sear undisturbed for 2–3 minutes until deeply caramelized and charred on the bottom. Flip and sear for another 2 minutes on the other side, or to your desired doneness.

Add the butter and smashed garlic to the pan and baste the steaks continuously for 1 minute as the butter foams. Remove from the pan and let rest for 5 minutes before slicing thinly against the grain.

In a separate skillet over medium heat, pour the green sauce directly into the skillet and warm gently for 2–3 minutes while stirring constantly. Do not let it boil or the vibrant green color will dull and the basil will lose its brightness.

Add the drained pasta directly to the skillet and toss aggressively to coat every strand, adding reserved pasta water a generous splash at a time until the sauce reaches a perfectly silky consistency that clings to every strand.

Transfer the pasta to a large serving platter. If serving with steak, fan the sliced steak generously over the top. Finish with a generous amount of freshly grated Parmesan and serve immediately.