Make the sauce by finely chopping the parsley and garlic, then combine both with the remaining ingredients. Cover and refrigerate to let the flavors mingle, a few hours or overnight.
Around 30 minutes to 2 hours ahead of when you want to cook, place the steak in a freezer bag or plastic container. Add two tablespoonfuls of the chimichurri and the additional 1 tablespoon of oil and lemon juice and spread all over the steak. If you are using a box, you can spread 1 tablespoon over each side of the steak, add the oil and lemon and then spread on both sides to be well coated.
When ready to cook
Pre-heat the grill or a grill pan to a medium-high heat. Let the steak come to room temperature then cook for around 3 - 5 minutes on each side, depending on how thick the piece of steak is and how well done you prefer the steak.
Once cooked, let the steak sit for around 5 minutes on a cutting board, tented with foil if you prefer, before slicing, across the grain of the meat.
Video
Notes
This recipe should give you plenty sauce for serving - it's also great over vegetables like potatoes and grilled portobello mushrooms (as here) and zucchini. If you have leftover, you can store in the fridge for a few days or freeze.