Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Cut the red potatoes into 1/2-inch cubes and toss them on the baking sheet with half of the olive oil, the dried rosemary, and a pinch of the salt and pepper.
Roast the potatoes for 20-25 minutes, flipping halfway through, until they are golden brown and crispy.
While the potatoes roast, pat the steak dry with a paper towel and season both sides with the remaining salt and pepper.
Heat a cast-iron skillet over medium-high heat with the remaining olive oil.
Once the pan is hot, sear the steak for 3-4 minutes per side for medium-rare, adding the minced garlic and dried thyme to the pan during the final minute to baste the meat.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes before slicing against the grain.
Remove the potatoes from the oven and immediately toss them with the crumbled feta cheese and fresh chopped parsley while still hot.
Serve the sliced garlic-herb steak alongside the feta-roasted potatoes.