Cook shells in boiling water for 9 minutes. Drain, rinse under cool water to stop cooking and place on cookie sheet to cool.
Meanwhile, melt butter in large skillet and add mushrooms. Saute until browned. Remove from pan to medium mixing bowl. Add spinach and garlic to hot skillet along with about 1/4 cup of water and saute until wilted. Drain well in a colander, pressing with the back of a spoon to remove as much moisture as possible. Chop and add to mushrooms.
Combine ricotta, half of Parmesan, and half of Mozzarella with spinach and mushrooms. Season with salt and pepper. Mix well.
Place 1/8 cup (2 Tbs) of filling in each shell. At this point, shells can be placed on a parchment paper lined cookie sheet and frozen just until they are solid enough to be placed in a ziploc bag and then stored in freezer.
To continue cooking, Pour half of marinara sauce into the bottom of a baking dish. You will only need the full 24 oz if you're cooking all of the shells. For just a few shells, use whatever amount of sauce you like. Place shells in pan, cheese size up. Pour the rest of the sauce (or less for fewer shells) over the shells. Sprinkle remaining parmesan cheese and italian cheese over the shells.
Cover with foil and bake at 400 deg F. for 25 minutes. Remove foil and bake until cheese is bubbly.
Frozen shells: Take out of freezer the night before and thaw. Then bake as usual. May have to add 10 minutes as they'll be cold from the fridge.
NOTE: I use 1 oz of marinara for each shell and nutritional info is calculated on that. Also 4.72gm of italian cheese per shell and 1.5gm of parmesan per shell