Allow steak to come to room temperature on the bench, at least 30 mins, before cooking. Preheat barbecue to high. Whisk together olive oil, red wine vinegar and sugar in a large bowl. Add shallots and tomato, stir well. Add rocket and basil, toss to combine.
Pat steak dry with paper towel, drizzle both sides with oil, and sprinkle with a good pinch of sea salt. Place on preheated barbecue and cook 12 minutes. Turn and cook a further 10–12 minutes. Transfer steak to a plate, cover loosely with foil, and rest for 10 mins.
While the steak is resting, melt butter in a skillet over medium heat. Add sage and cook, stirring, until leaves start to crisp. Remove from heat and drizzle over beef. Serve steak and sage butter with salad.