Discard sausage casings. In a 10- to 12-inch frying pan over medium-high heat, stir and crumble sausage until lightly browned, 5 to 10 minutes. Discard any fat. Let cool 10 minutes.
Meanwhile, in a 5- to 6-quart pan over high heat, bring 3 quarts water to a boil. Add manicotti and cook until barely tender to bite (test 1 at edge), 5 to 8 minutes. Drain and immerse in cold water until cool, about 2 minutes, then drain.
To sausage, add spinach, ricotta, parmesan, and nutmeg; mix, and add salt to taste. In a small bowl, beat eggs to blend. Stir into sausage mixture.
Spoon 1 cup tomato sauce evenly into each of 2 foil pans (9 by 13 in.).
With a spoon, fill manicotti shells equally with spinach mixture (about 1/4 cup in each). Arrange in a single layer in sauce, and spoon remaining sauce evenly over manicotti.
Drizzle 1/2 cup water over pasta in each pan. Seal pans with foil, and freeze (see Cold Facts below).
Thaw 1 pan (see notes). Bake, covered, in a 375° oven until steaming in center (cut to test), 50 to 55 minutes. Season to taste with salt and pepper.
Cold Facts. Packaging: foil pans. Seal foil pans with a double layer of foil to protect foods for storage. Label each dish with permanent marker. Make a master list of the dishes. As you remove each from the freezer, cross it off the list. For best quality, freeze meat mixtures no longer than six weeks, others three months.