1 serrano pepper, seeded and chopped
3 cloves garlic, smashed
2 small red onions, divided
1 1/2 lb beef flank steak
1 1/2 cups Italian parsley leaves
1/2 cup fresh cilantro leaves
1/4 fresh oregano leaves
1/2 teaspoon kosher salt
1/4 cup olive oil
1/4 cup red wine vinegar
1/4 cup water
2 Deli pita bread
Score both sides of steak in a diamond pattern. Place steak in a large zip-top bag.
Place parsley, cilantro, oregano, chopped red onion, serrano pepper, garlic, and salt bowl of food processor. Cover and pulse until roughly chopped. Add oil and vinegar; process until smooth.
Divide chimichurri and chill half. Add water and remaining chimichurri to steak. Seal bag and turn to coat. Refrigerate 4 hours (or overnight) to marinate.
Preheat grill on medium. Drain steak; discard marinade. Place steak on grill; cover and grill 8–10 minutes. Turn steak and add onions to grill; cook 8–10 minutes until onions are tender and steak is 145°F and lightly charred, turning once halfway through grilling. Transfer steak and onions to cutting board; let stand 5 minutes. Add pita bread to grill; cook 2–3 minutes or until warm. Slice steak thinly across the grain. Cut pita bread rounds in wedges. Serve steak with reserved chimichurri, grilled onion, and pita.