Preheat oven to 375F.
Cutting lengthwise, remove the top off the loaf of bread and scoop out the insides. Reserve the top piece.
Bring about 2 inches of water to a boil in a deep skillet over high heat. Trim the ends off the broccoli rabe and cut into 3 to 4-inch pieces. Cook in salted water and reduce heat to simmer. (Add the rabe in stages as it wilts down its a mountain of greens!) Simmer 5 minutes then drain. Shock in cold water and reserve.
Heat the EVOO, 3 turns of the pan, over medium to medium-high heat. Add sausage and break it up into small pieces as it browns. Add the garlic, cook 2 minutes then add the reserved rabe and cook to heat through. Season with salt, pepper and nutmeg, to taste.
Fill the hollowed-out loaf of bread with the sausage and rabe mixture, and top with a the three cheeses: Parmigiano-Reggiano, mozzarella and provolone. Replace the top of the loaf, wrap bread in foil and bake to melt cheese, about 12-15 minutes. Cut into thick pieces and serve.