First make the chimichurri salsa. Put the onion, garlic, chili and vinegar in a bowl and set aside.
Meanwhile, heat a griddle or frying pan until hot. Mix 1 teaspoon of olive oil with the chili powder and some seasoning. Rub the sweetcorn cob all over with the spiced oil. When hot, add the corn to the griddle pan and cook for 10-15 minutes, turning every couple of minutes until lightly charred and cooked through. Transfer to a board and leave to cool for a few minutes.
Return pan to heat. Brush each steak all over with 1/2 teaspoon oil and season. When hot, add the steaks and fry over a high heat for 3 minutes, turn over and cook for a further 2-3 minutes. This should be medium rare, but will depend on the thickness of the steaks. Cook for longer to desired doneness. Transfer to a warm plate and set aside to rest for a few minutes.
Add the parsley, oregano and remaining oil into the bowl with the salsa. On a clean board, firmly hold the cooled sweetcorn cob on its end and use a sharp knife to cut away the charred kernels. Add these to the salsa, along with the diced avocado flesh and cherry tomatoes. Toss together and season to taste.
Serve steaks with chimichurri salsa.