Preheat oven to 400ºF. Pat sirloin dry with paper towels.

Heat oil over high heat in an oven-safe skillet. Season sirloin with salt and pepper to taste, add to the hot pan and sear about 1-2 minutes on each side, until browned. Once steak is seared, place skillet in oven for about 10-12 minutes, for a medium done steak, depending on the thickness. If you would like it more well done, leave it in a bit longer.

When the sirloin is cooked to your liking, remove from skillet onto a cutting board and allow to rest for about 5 minutes. Using the same skillet, add onions and sauté for about 3 minutes, until they begin to become tender. Add mushrooms and sauté for about 5 minutes, until tender.

Assemble salad in a large family style serving bowl. Add the spinach, dates, avocado, cooked onions and mushrooms. Top with thinly sliced steak and drizzle with your favorite dressing.