In a small bowl, whisk together all the dressing ingredients, except the oil.
Slowly whisk in the oil to incorporate, pour about 2 large tablespoons of the vinaigrette over the steak and mix; refrigerate the rest until needed.
Heat a medium sauté pan over high/medium-high heat with a little oil in it.
Add the mushroom slices, season with salt and pepper, and brown on both sides.
Add the steak and sauté until browned.
Place the European mixed greens on a large plate or bowl, sprinkle the onions over the top, pour the steak and mushrooms in the center, places the mandarin orange sections around the steak and mushrooms, sprinkle with blue cheese and salt and pepper, pour on dressing.