Preheat oven to 375°F. Slice off the top half-inch of the bread and reserve. Scoop out the insides of the loaf so the shell's about a half-inch thick and cut the scooped bread into cubes.
In a large skillet, heat 6 tablespoons olive oil over medium heat. Add garlic and rosemary, and cook 2 minutes. Place the bread shell on a baking sheet and 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 cubes to the baking sheet, scattering them around the bread shell. Place the reserved bread top on the baking sheet cut side up. Bake until everything's brown and toasted, 10-15 minutes.