Prepare the marinade for the steak by first cooking the chilies, garlic and cumin seeds in a dry pan over medium heat until the garlic and chilies are lightly charred and the cumin is fragrant, about 5 minutes.
Add the cooked chilies, garlic and cumin to a food processor along with the rest of the ingredients, aside from the oil, and process until everything is very finely chopped. Add in the olive oil and pulse to emulsify completely.
Pour the marinade into a ziptop bag with the steak and make sure the marinade coats the meat completely. Pop the bag in the fridge for at least 1 hour, and up to 6 hours, turning to make sure the meat is coated in the marinade about halfway through the marinating time. Before cooking the steak, remove it from the fridge and let it hang out at room temp for at least 15 minutes, preferably about half an hour.
To cook the steak (for medium-rare preparation) drain off the excess marinade and heat a dry grill pan over medium-high heat until the pan is very hot, and grill the steak for 6 minutes on the first side. Flip the steak and grill for another 5 minutes. Place the steak on a cutting board or counter and tent with foil for 10 minutes before slicing and serving.
Pop all of the ingredients, aside from the olive oil, into a food processor and blend until everything is finely chopped.
At this point, you can add the oil and continue blending to emulsify the ingredients (which is a less traditional method) or you can put the chimichurri in a bowl and then add the oil, stirring to combine the ingredients together, which is a more traditional method. Following either preparation, I like to stir in an additional teaspoon of red chili flakes, but that's completely optional! Serve immediately.