Preheat the oven to 375°. Slice off the top 1/2 inch of the bread; reserve. Scoop out the insides of the loaf, leaving a 1/2-inch-thick shell, and cut into cubes.
In a large skillet, heat 6 tablespoons olive oil over medium heat. Stir in the garlic and rosemary for 2 minutes. Place the bread shell on a baking sheet; brush the inside with some of the seasoned oil. Add 3 cups bread cubes to the skillet with the remaining seasoned oil and toss to coat; transfer to the baking sheet, surrounding the bread shell. Reserve the skillet. Place the bread top on the baking sheet cut side up. Bake until browned and toasted, 10 to 15 minutes.
Meanwhile, in the reserved skillet, heat the remaining 2 tablespoons olive oil over medium-high heat. Add the onions and cook, stirring, until golden, 10 minutes; transfer to a bowl. Add the steak to the skillet and cook for 4 minutes; season with salt and pepper. Add the wine and cook for 2 minutes. Stir in the onions.
Spoon half of the steak-onion mixture into the bread bowl; top with half of the cheese, then half of the croutons. Layer with the remaining steak-onion mixture, croutons and cheese. Bake until the cheese has melted, 3 to 5 minutes. Top with the bread lid. To serve, slice into wedges.