Take the steak out of the fridge and let it sit at room temperature for 30 minutes to warm up a little.
Heat a griddle or skillet until smoking hot. Drizzle the oil onto the steak and sprinkle the salt and pepper onto both sides. Use your hands to massage the seasoning and oil in. Place on the hot griddle and cook for 2 minutes on each side (cook for longer if you prefer the centre less pink). Take off the griddle and place on a chopping board. Allow the steak to rest whilst you prepare the rest of the salad.
Place the chimichurri ingredients in a mini chopper or food processor and pulse until blended together (alternatively you can just chop everything finely by hand and mix together). Put to one side.
Split the bag of mixed salad between two large bowls. Top with the cherry tomatoes, avocado chunks, red onion and cucumber.
Slice the steak very thinly and arrange the slices on top of the two salads. Drizzle over the chimichurri and serve.