Combine parsley, oregano, garlic, vinegar, lemon juice, and red pepper flakes in a bowl. Whisk in olive oil until blended. Set aside half for serving.

Marinate the flank steak in the remaining sauce for at least 30 minutes.

Preheat your grill to medium-high heat (around 450°F).

Remove the steak from marinade, pat dry, and season with salt and pepper.

Grill steak for 4-5 minutes on each side for medium-rare (internal temp 130-135°F).

Let rested steak sit for 5-10 minutes before slicing against the grain.

Serve sliced steak with reserved chimichurri sauce and additional herbs.