Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Wash and cube the red potatoes into bite-sized pieces, then toss them in a bowl with 0.5 tbsp olive oil, 0.125 tsp sea salt, and the dried rosemary.
Spread the potatoes in a single layer on the baking sheet and roast for 20 minutes until they begin to soften and turn golden.
Trim the woody ends off the asparagus and add them to the baking sheet with the potatoes, drizzling with 0.25 tbsp olive oil and another pinch of salt.
Return the vegetables to the oven for an additional 10 to 12 minutes until the asparagus is tender and the potatoes are crispy.
While the vegetables finish, season the sirloin steak on both sides with the remaining sea salt and black pepper.
Heat the final 0.25 tbsp of olive oil in a cast-iron skillet over medium-high heat and sear the steak for 4 minutes per side for medium-rare.
Add the minced garlic to the skillet during the last minute of cooking, tilting the pan to baste the steak with the aromatic garlic oil.
Remove the steak from the pan and let it rest for 5 minutes before slicing it against the grain to serve with the roasted vegetables.