Preheat a grill to medium-high heat.
In a bowl, combine the olive oil and rosemary.
2 tbsp. olive oil, 3 tbsp. chopped rosemary
Season both steaks with sea salt and freshly ground black pepper.
Sea salt and freshly ground pepper
Brush the steaks with the rosemary-olive oil mixture until well coated and let sit for 30 minutes.
2 ribeye steaks
While the steaks are sitting, melt some cooking fat in a skillet over medium-high heat.
Cooking fat
Add the onions and cook, stirring frequently, until browned and soft.
2 sweet onions
Add the mushrooms and cook for another 5 minutes, stirring constantly.
1 cup baby portobello mushrooms
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.
2 tsp. balsamic vinegar
Grill the steak for 4 to 5 minutes on each side and let rest for 5 to 8 minutes.
Serve the steaks topped with caramelized onions and mushrooms.