In a large bowl mix together the garlic, ricotta, egg, parmesan, 1 cup mozzarella, spinach, salt, pepper, Italian seasoning, and nutmeg until well combined.
Put the manicotti filling into a gallon sized zip-top bag.
Snip off one corner with scissors as you would with a pastry bag, squeeze the filling into each manicotti tube until each tube is full.
Place 3/4 of 1 jar of sauce over the bottom of the slow cooker. Lay a layer of filled manicotti on top of the sauce, cover the first layer of pasta with about 1/2 cup of sauce and repeat with the remaining pasta, ending with sauce covering the pasta completely.
Cook on high for 2 hours or low for about 4 hours. Check periodically to make sure it's not drying out. When the manicotti is fork tender, sprinkle the remaining 1 cup of mozzarella over the top and cook until melted about 10 to 15 minutes.