Step 1: Prepare the chimichurri sauce. In a bowl, combine finely chopped parsley, cilantro (if using), minced garlic, red wine vinegar, olive oil, dried oregano, red pepper flakes, lemon juice, salt, and black pepper. Mix well and let it sit at room temperature for 15 minutes to allow the flavors to meld.

Step 2: Preheat the grill or grill pan to medium-high heat. Brush both sides of the sirloin steak with olive oil. Season generously with salt, black pepper, and garlic powder if desired.

Step 3: Place the steak on the hot grill. Grill for 4–6 minutes per side for medium-rare, depending on thickness, or adjust cooking time to your preferred doneness. Use a meat thermometer to check internal temperature: 130–135°F for medium-rare, 140–145°F for medium.

Step 4: Remove the steak from the grill and let it rest for 5–10 minutes to allow juices to redistribute.

Step 5: Slice the steak against the grain into thin strips.

Step 6: Drizzle chimichurri sauce over the sliced steak or serve it on the side for dipping.