Preheat the oven to 375°. Cut off the top quarter of the bread lengthwise and scoop out the bottom piece.
In a deep skillet, bring about 2 inches of water to a boil. Salt the water and add the broccoli rabe in batches, simmering for 5 minutes. Drain and rinse under cold water.
Wipe out the skillet; add the EVOO, 2 turns of the pan, and heat over medium heat. Add the sausage and cook, crumbling with a spoon, until browned, about 8 minutes. Add the garlic and cook for 2 minutes. Add the broccoli rabe and season with salt, pepper and nutmeg, if using.
Fill the bread shell with the sausage mixture. Top with the provolone, parmigiano, mozzarella and bread lid. Wrap the sandwich in foil and bake until the cheese is melted, 15 minutes. Cut into thick slices to serve.