Let steak stand at room temperature for 30 minutes.
In a small bowl, stir together brown sugar, chili powder, garlic powder, salt, pepper, mustard, and cumin. Generously sprinkle mixture all over steak; let stand 15 minutes more.
Heat a large cast-iron grill pan over medium-high heat; spray pan with cooking spray.
Cook steak, turning once, until an instant-read thermometer inserted in the thickest part registers at least 130°, 10 to 15 minutes, or until desired doneness. Transfer to a cutting board, and let stand for 10 minutes. Slice steak diagonally against the grain. Serve with Chimichurri Sauce.
Chimichurri Sauce
In a medium bowl, whisk together all ingredients. Cover and let stand at room temperature for at least 15 minutes before serving.
Recipe by Southern Cast Iron at https://southerncastiron.com/grilled-flank-steak-chimichurri-sauce/