Puree garlic, parsley, cilantro, red wine vinegar, olive oil, salt, pepper and cumin in a food processor or blender until blended. Scrape down sides and continue blending until desired consistency.
Add ¼ cup chimichurri sauce and Flank Steak to bag, seal, and shake until steak is evenly coated. Marinate in the refrigerator up to 24 hours. Store remaining sauce in an airtight container.
When ready to cook, heat grill to medium heat then place steak directly on grid and grill, covered, 8 to 10 minutes per side or until a meat thermometer registers 145°F for medium rare or 160°F medium doneness.
Remove from heat; let stand for 5 minutes. Cut the steak diagonally across grain into thin slices. Serve with remaining chimichurri sauce.
Store the chimichurri sauce in an airtight container up to 1 week.
Cook steak until desired degree of doneness using a meat thermometer to check temperature: