Preheat grill to high.
To prepare salad: Combine tomatoes, onion, oil and vinegar in a medium bowl. Season with salt and pepper.
To prepare steak: Rub steak with oil. Season on both sides with salt and pepper. Grill the steak 3 to 4 minutes per side for medium-rare. Allow the steak to rest 5 minutes; serve with the salad on the side and a dollop of chimichurri sauce on top.
Meat-Buying Tips: Make sure that packaged meat isn't past its sell-by date and that there's not much moisture in the packaging.Touch it if possible - it should be firm and not soft. Look for bright red (not gray) meat. Vacuum-packed meat will be darker looking and should turn red as soon as it's exposed to air.