Brown the ground meat first in a separate skillet, add the pizza sauce and stir to combine.
Put 1/2 of the meat-sauce into the crockpot, followed by 1/2 the fresh spinach, 1/2 the pepperoni, 1/2 of the olives, 1/2 of the mushrooms, 1/2 the onion, and 1/2 the Mozzarella cheese.
Repeat the layers using up the remaining ingredients, ending with the Mozzarella cheese covering the top.
Put the crockpot setting on LOW, cover, and cook for 5 to 6 hours.
Turn off the heat and let it sit for 15 minutes before cutting.