Preheat your oven to 400°F and line a baking sheet with parchment paper.
Dice the baby potatoes into bite-sized pieces and toss them with half of the olive oil, chopped rosemary, and a pinch of sea salt.
Spread the potatoes on the baking sheet and roast for 25 minutes or until they are golden brown and crisp.
While the potatoes roast, season the sirloin steak generously on both sides with salt and cracked black pepper.
Heat a heavy cast-iron skillet over medium-high heat and add the remaining olive oil.
Place the steak in the hot skillet and sear for 3 to 4 minutes per side for medium-rare, then remove from the pan and let it rest on a cutting board.
In the same skillet, lower the heat slightly and add the minced garlic, sautéing for 30 seconds until fragrant.
Add the fresh spinach to the pan and toss constantly until just wilted, then remove from heat immediately.
Slice the steak against the grain and serve alongside the roasted potatoes and sautéed spinach.