Bring a small pot of water to a boil. Peel and cube the yellow potato into 1-inch pieces.
Boil the potato for 12-15 minutes until fork-tender, then drain thoroughly.
Mash the potato with the unsweetened almond milk, one minced garlic clove, and half of the ghee until smooth and creamy.
Pat the sirloin steak completely dry with paper towels and season both sides with sea salt and black pepper.
Heat a cast-iron skillet with the olive oil over medium-high heat until the oil is shimmering and just starting to smoke.
Place the steak in the skillet and sear for 3-4 minutes per side for medium-rare.
During the final minute of cooking, add the remaining ghee, the second smashed garlic clove, and the rosemary sprig to the pan, spooning the melted ghee over the steak.
Remove the steak from the pan and let it rest for 5 minutes before slicing against the grain and serving over the garlic mashed potatoes.