Pat the sirloin steak dry thoroughly with paper towels. Season both sides with 1/4 tsp salt and 1/8 tsp black pepper. Let it rest at room temperature for at least 10 minutes (while you prepare zucchini), up to 30 minutes, for best results.

Heat the large cast iron skillet over medium-high heat. Add 1 tbsp olive oil. Once shimmering, add the zucchini pieces in a single layer. Season with the remaining 1/4 tsp salt and 1/8 tsp black pepper.

Cook the zucchini for 3-4 minutes, stirring occasionally, until slightly tender-crisp and lightly browned. Remove the zucchini from the skillet and set aside on a plate.

Return the skillet to medium-high heat. If needed, add another teaspoon of olive oil. Carefully place the seasoned steak into the hot skillet. Sear for 3-5 minutes per side for medium-rare, or longer for desired doneness (e.g., 5-7 minutes per side for medium).

Reduce heat to medium-low. Add the unsalted butter and minced garlic to the skillet. As the butter melts, tilt the pan slightly and use a spoon to baste the melting garlic butter over the steak for 1-2 minutes, flipping once.

Remove the steak from the skillet and place it on a clean cutting board. Let it rest for 5-10 minutes (crucial for juiciness).

While the steak rests, add the beef broth (or water) to the skillet with the remaining garlic butter. Bring to a simmer, scraping up any browned bits from the bottom of the pan. This creates a flavorful sauce.

Return the cooked zucchini to the skillet with the sauce and toss gently to coat. Cook for an additional 1-2 minutes until heated through.

Slice the rested steak against the grain into desired thickness. Divide the sliced steak and garlic butter zucchini between two plates. Drizzle any remaining pan sauce over the steak and zucchini. Garnish with fresh chopped parsley.