Grill over gas or charcoal until grill marks form.
Coat steak with blackening spice. In large skillet, heat 1 teaspoon oil; sear steak on both sides to desired doneness. Remove steak from pan and let rest.
Wipe out pan, heat 2 teaspoons oil. Add onions and garlic; season with salt and pepper. Sauté until soft, but not browned. Add spinach and sauté just until spinach starts to wilt.
Mound spinach on 4 plates, dividing it equally. Carve steak into thin slices; arrange on top of spinach. Top with potatoes and sprinkle with bleu cheese. Serve with bleu cheese dressing on the side, if desired.