Steam potatoes 10 to 12 minutes or until tender, drain and cool. Cut potatoes in half.
Coat steak on both sides with blackened seasoning. Heat 1 teaspoon oil in a large skillet over medium-high heat; add steak and cook 5 minutes on each side or to desired doneness. Remove steak from pan and set aside.
Wipe out pan then add remaining 2 teaspoons oil. Add onions and garlic. Sauté over medium heat for 10 minutes or until soft, but not browned. Add spinach and sauté just until spinach starts to wilt.
Place equal amounts of spinach on 4 plates. Thinly slice steak and arrange on top of spinach. Top with potatoes and sprinkle with bleu cheese. Serve with bleu cheese dressing on the side, if desired.