In a small bowl, combine the lemon zest, black pepper, salt, garlic powder, onion powder, mustard powder, and sugar. Mix well. Season both sides of the steaks with the lemon-pepper mixture, pressing the coating into the steaks. Let the steaks stand at room temperature for 15 minutes.

Brush a large grill pan or griddle (or outdoor grill) with the olive oil and preheat to medium-high. Add the steaks and cook for 2 to 3 minutes per side for medium-rare, or longer for more fully cooked meat. If desired, place some lemon slices on the pan to cook alongside the steak.

To test for doneness, use a meat thermometer. For medium-rare steaks (soft, dark pink inside), the final temperature is 145 degrees, so pull the steaks at about 140 degrees (which is what I did for this recipe). For medium (soft, some pink inside), the final temp is 160 degrees, so pull the steaks from the heat around 155 degrees. For well done (very firm, no pink inside), the final temp is 170 degrees, so pull the steaks from the heat around 165 degrees.

Transfer the steaks to a plate and tent with foil for 5 minutes. Top with green onions, if using, and serve.