You will need:
* 3/4 pound flat iron steak
* 1 medium onion, sliced
* 2 teaspoons butter
* 1/3 cup plus two teaspoons olive oil
* 2 tablespoons red wine vinegar
* 1 tablespoon Dijon mustard
* 1/2 teaspoon garlic powder
* 2 teaspoon Worcestershire sauce
* 1/4 teaspoon kosher salt (plus more for seasoning steak)
* 1/4 teaspoon pepper (plus more for seasoning steak)
* 2.5 ounces baby romaine leaves
* 2.5 ounces baby spinach leaves
* 1 cup thinly sliced crimini mushrooms
1. Preheat your grill to medium heat.
2. Season both sides of the steak with salt and pepper.
3. Heat butter and olive oil in a skillet over medium heat. Once butter has melted add onions. Cook, stirring occasionally, until onions have caramelized. About 20 minutes. Remove from heat.
4. After you start the onions, begin cooking your steak. For medium rare, cook for 10 minutes. Transfer to a plate and cover with foil, allow to rest for 10 minutes, the steak will continue to cook during this time. If you would like it more well done cook 15 or 20 minutes before resting.
5. While the steak is cooking and the onions are caramelizing, whisk together olive oil, vinegar, mustard, garlic powder, Worcestershire sauce, 1/4 teaspoon kosher salt and 1/4 teaspoon pepper together in a bowl. Set aside.
6. Place romaine lettuce, spinach and sliced mushrooms in a salad bowl.
7. After steak has rested, slice thinly against the grain. Place sliced steak on top of lettuce. Top with caramelized onions. Drizzle with dressing and toss to coat. Serve.