Halve eggplants lengthwise. Scoop out the flesh leaving the shells 1/8-inch thick. Salt the shells and invert to drain over paper towels for 30 minutes. Salt the flesh and drain it in a strainer.
Defrost the spinach in a microwave, squeeze out all of the water and separate it with fingertips. Put the spinach in a mixing bowl.
Heat the oven to 350 degrees F.
Heat a medium skillet over medium-high heat with extra-virgin olive oil, a turn of the pan. Add the meat, sage and salt and pepper, to taste. Lightly brown the meat, then stir in the garlic and cook for 30 seconds, Add the meat mixture to the mixing bowl.
Melt 4 tablespoons butter in a sauce pot over medium heat. Whisk in the flour, cook for 1 minute, then whisk in milk and bring the mixture to a bubble. The sauce should be thick enough to coat the back of a spoon. Season the sauce with salt, pepper and nutmeg, to taste. Cool for a few minutes, then add it to the meat and spinach. Pat the salted flesh of the eggplant dry and finely chop, then stir it into meat mixture along with an egg and about 1/2 cup cheese.
Tear the bread, add it to a food processor and pulse-grind into fresh crumbs. Finely chop the parsley leaves and stir it into the bread crumbs.
Pat the eggplant shells dry and drizzle a baking dish with extra-virgin olive oil. Arrange the eggplant shells in the dish and mound the filling into them. Top with bread crumbs and a little more cheese, dot with dabs of butter and bake until lightly golden, about 30 minutes. Cool completely, cover and refrigerate for a make-ahead meal. To reheat, put the eggplant in a 350 degree F oven, loosely covered, for 20 minutes, then uncover and bake until deeply golden in color, about 10 to 15 minutes. Serve a whole eggplant per person.