Combine oil, garlic, vinegar, salt, cumin, red pepper flakes, black pepper, cilantro, oregano, and parsley in a blender.
Pulse blender 2 to 3 times; scrape down the sides using a rubber spatula. Repeat pulsing and scraping process until a thick sauce forms, about a dozen times. Refrigerate until use. It gets better as it sits a couple hours.
For the London broil,
If you're using the marinade, marinate the London broil for a few hours or overnight. Discard the marinade & bring the meat to room temperature. Pat the meat with a paper towel to complete dry it off & then season with garlic salt & pepper to taste.
Heat a cast iron skillet over medium high heat until it is smoking hot. Don't add any butter or oil to it. Cook the London broil for 7-12 minutes on each side. Closer to 7-8 if you want yours rarer (like mine) or closer to 10-12 minutes if you want yours more well done. Once it's seared on both sides, remove from the skillet onto another plate. Let the London broil sit for 10-15 minutes before slicing. Slice thin & against the grain for best results. Serve with the chimichurri sauce.