Place the sirloin in a flat dish. Whisk together the olive oil, red wine vinegar and garlic in a measuring cup. Pour the marinade over the steak and lift the steak up so the marinade covers the bottom side of the steak as well. Let sit for 1 hour at room temperature, or 2-3 hours in the fridge. Heat the grill to medium (or you can use an indoor grilling pan). Thread the meat onto skewers and grill until medium-rare to medium, depending on your preference. Set aside.
Place the romaine, red onion, Kalamata olives and feta cheese, cucumber and tomatoes in a salad bowl. Whisk the olive oil, red wine vinegar, garlic, and sea salt in a measuring cup. Pour the dressing over the salad. Serve the salad with the beef kabobs and season with just a bit of freshly ground black pepper.