Preheat oven to 400ºF with a rack in the center position.

Using a very sharp knife, carefully cut flank steak almost in half horizontally; do not cut all the way through. Open like a book. Spread evenly with olive oil, then crushed garlic; sprinkle with Creole seasoning, onion powder, and garlic powder. 

Drain thawed spinach and squeeze out as much moisture as possible; distribute spinach evenly over flank steak, leaving ½-inch clear on the two long sides. Distribute blue cheese evenly over spinach. 

Starting with a long side, tightly roll up flank steak; tie at 1-inch intervals with kitchen twine. Transfer to a baking dish and roast for 40 minutes. Move steak to a rack set 6 inches from the heat source, set oven to broil, and broil, turning occasionally, browned on all sides, about 10 minutes. Remove from the oven, tent loosely, and allow to rest 10 minutes; slice across the rolled steak and serve.