Preheat oven to 400°F and line a baking sheet with parchment paper.
Scrub the Yukon gold potato and dice into 1/2-inch cubes, then toss with half of the olive oil and a pinch of the sea salt and black pepper.
Spread potatoes on the baking sheet and roast for 20-25 minutes until they are golden and crispy.
While potatoes roast, thinly slice the cucumber and toss in a small bowl with the lemon juice and a pinch of sea salt.
Mince the garlic and finely chop the fresh rosemary and parsley.
Season the sirloin steak on both sides with the remaining sea salt, black pepper, and the chopped rosemary.
Heat the remaining olive oil in a cast-iron skillet over medium-high heat and sear the steak for 3-4 minutes per side for medium-rare.
During the final minute of searing, add the minced garlic to the pan and baste the steak with the fragrant oil.
Remove the steak from the heat and let it rest for 5 minutes before slicing against the grain.
Plate the sliced steak with the roasted potatoes and cucumber salad, then garnish the entire dish with fresh parsley.