Preheat your oven to 400°F and line a baking sheet with parchment paper.
Carefully halve the spaghetti squash lengthwise, scoop out the seeds, and brush the flesh with half of the olive oil and a pinch of salt.
Place the squash cut-side down on the baking sheet and roast for 35 to 40 minutes until the shell is easily pierced with a fork.
While the squash roasts, season the sirloin steak generously on both sides with the remaining sea salt and black pepper.
Heat a heavy cast-iron skillet over medium-high heat with the remaining olive oil until shimmering.
Place the steak in the hot skillet and sear for 4-5 minutes per side for medium-rare doneness.
During the last 2 minutes of cooking, add the minced garlic and rosemary to the pan, tilting the skillet to baste the steak with the infused oil.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes before slicing against the grain.
Use a fork to scrape the roasted squash into spaghetti-like strands and toss them with the fresh chopped parsley.
Divide the squash strands between plates and top with the sliced steak and any remaining pan juices.