Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Dice the Yukon Gold potato into 1/2-inch cubes and place them on the prepared baking sheet.
Drizzle the potatoes with 0.25 tbsp of the olive oil and season with a pinch of the sea salt and black pepper.
Roast the potatoes for 20-25 minutes, tossing halfway through, until they are golden-brown and crispy on the edges.
While the potatoes roast, finely mince the fresh parsley, cilantro, and garlic clove.
In a small bowl, whisk together the minced herbs, garlic, red wine vinegar, the remaining 0.5 tbsp of olive oil, and the red pepper flakes to create the chimichurri.
Season the sirloin steak on both sides with the remaining sea salt and black pepper.
Heat a heavy cast-iron skillet over medium-high heat until it is very hot and begins to slightly smoke.
Place the steak in the skillet and sear for 3-4 minutes per side for a perfect medium-rare finish.
Remove the steak from the skillet and let it rest on a cutting board for 5 minutes to allow the juices to redistribute.
Slice the steak against the grain and serve it topped with the vibrant chimichurri sauce alongside the roasted potatoes.