Place parsley, onion, tomato, bell pepper, garlic, oregano, paprika, salt, bay leaf, black pepper, and chile flakes into a large bowl. Allow to sit for 30 minutes.
Mix vinegar and water into the chimichurri and allow to sit for another 30 minutes.
Mix olive oil into the mixture and transfer to a glass, non-reactive, container with a lid. Make sure liquid covers the ingredients; if not, add equal parts water, red wine vinegar, and oil until it does. Place in the refrigerator and let stand, 6 hours, up to 2 days.
Poke steaks all over with a fork. Mix sweet onions, cilantro, white wine vinegar, olive oil, garlic, thyme, oregano, and cumin together for beef in a bowl.
Place steaks into marinade and refrigerate for 6 hours, to overnight.
Preheat an outdoor grill for high heat and lightly oil the grate.
Sear steaks on the preheated grill, about 2 minutes on each side. Reduce heat to medium and grill until desired doneness level, or about 5 minutes per side. Serve with chimichurri.