Preheat the oven to 400°F and line a small baking sheet with parchment paper.
Dice the Yukon Gold potato into 1/2-inch cubes and toss with 0.5 tbsp of melted ghee, half of the salt, and half of the pepper.
Spread the potatoes in a single layer on the baking sheet and roast for 20-25 minutes until golden and crispy.
While the potatoes roast, pat the sirloin steak dry and season both sides with the remaining salt and pepper.
Heat a cast-iron skillet over medium-high heat and add the remaining 0.5 tbsp of ghee.
Place the steak in the hot skillet along with the smashed garlic clove and rosemary sprig, searing for 3-4 minutes per side for medium-rare.
Remove the steak from the pan and let it rest on a cutting board for 5 minutes.
In the same skillet, crack the egg and cook for 2-3 minutes until the whites are set but the yolk remains runny.
Slice the steak against the grain and serve alongside the roasted potatoes, topped with the sunny-side-up egg.