Bring a large pot of salted water to a boil and cook the spaghetti according to package directions until al dente.
While the pasta cooks, pat the sirloin steaks dry with paper towels and season both sides evenly with sea salt, black pepper, and garlic powder.
Heat the olive oil in a large cast-iron skillet over medium-high heat until shimmering.
Place the steaks in the hot skillet and sear for 4-5 minutes per side for medium-rare, or until they reach your desired level of doneness.
Remove the steaks from the pan and let them rest on a cutting board for at least 5 minutes to keep them juicy.
Drain the spaghetti, reserving a small splash of pasta water, then return the noodles to the pot.
Toss the warm spaghetti with grass-fed butter, parmesan cheese, and the reserved pasta water until the noodles are evenly coated and glossy.
Slice the rested steak against the grain and serve alongside the buttered spaghetti, garnished with freshly chopped parsley.