1. Preheat a grill to medium-high heat.

2. In a bowl, combine the olive oil and rosemary.

3. Season both steaks with sea salt and freshly ground black pepper.

4. Brush the steaks with the rosemary-olive oil mixture until well coated and let sit for 30 minutes.

5. While the steaks are sitting, melt some cooking fat in a skillet over medium-high heat.

6. Add the onions and cook, stirring frequently, until browned and soft.

7. Add the mushrooms and cook for another 5 minutes, stirring constantly.

8. Drizzle the balsamic vinegar into the pan. Stir around to scrape the bits up from the bottom of the pan. Remove from direct heat but keep warm.

9. Grill the steak for 4 to 5 minutes on each side and let rest for 5 to 8 minutes.

10. Serve the steaks, topped with the caramelized onions and mushrooms