Onions: In a bowl combine together water, vinegar, sugar, and salt. Next, submerge the sliced onions in the mixture and place it n the refrigerator for at least 30 minutes.
Vinaigrette: Combine shallots, vinegar, lime juice, honey, mustard and basil in a large bowl and slowly whisk in olive oil until completely combined. Finish by whisking in salt and pepper, and set aside.
Steak: Preheat the grill to high heat, in between 450° and 550°.
Coat the steak on all sides with olive oil, salt, and pepper and cook on a hot grill (450° to 550°) for 3 to 4 minutes on each side to reach a medium-rare internal temperature. Once cooked, remove from the heat and rest on a cutting board for 2 to 3 minutes before slicing.
Salad: In a large bowl or platter combine lettuce, tomatoes, cucumbers, watermelon, Feta cheese, radishes, and green onions. Place on steak slices and serve alongside dressing.