Make the chimichurri: In a mixing bowl, combine chopped parsley, cilantro (if using), garlic, red wine vinegar, red pepper flakes, oregano, olive oil, salt, and pepper. Stir well and let it sit to develop flavor.
Prep the steak: Pat steak dry and season with salt and pepper. Let it sit at room temperature for 10 minutes.
Heat the skillet: Place a cast-iron skillet over medium-high heat. Once hot, add olive oil.
Sear the steak bites: Add steak cubes in a single layer (work in batches if needed). Sear 2–3 minutes per side until browned and medium-rare inside. Remove from pan.
Toss with chimichurri: Let the steak rest for 2 minutes, then toss with chimichurri sauce or drizzle it over the top.
Serve: Garnish with extra herbs if desired and serve warm as an appetizer or over rice, salad, or roasted veggies.