Preheat the oven to 400 degrees.
Fill a large stock pot with salted water and boil pasta for 8-9 minutes. Do not over cook or they will break while stuffing. Cook a few extra shells because they tear easily.
Drain pasta and rinse with cold water to stop cooking.
While shells are boiling, chop your mushrooms and garlic. If needed, place spinach in the microwave on "Defrost" for 1-2 minutes. Squeeze excess water from spinach. Set aside.
In a large pan, add 1 TBSP olive oil. Sauté mushrooms about 3-4 minutes until browned around the edges. Do not move them often- let the mushrooms sear on the bottom of the pan to allow for browning. Add in TWO of the chopped cloves of garlic. Sauté until fragrant- about 30 seconds. Stir in the 1/2 cup of wine to deglaze the pan slapping up any brown bits. Add in salt, pepper and nutmeg. Add in spinach and cook on medium low until the moisture has completely evaporated. About 2 minutes.
Set mixture aside in a medium-sized bowl to cool.
In the same pan over medium low heat, melt the butter until frothy. Stir in the red pepper flakes, oregano and remaining garlic. Stir until spices bloom and butter becomes foamy. Pour in the marinara sauce and heat through about 3 minutes on a very low simmer.
Pour half of the sauce into a 9 x 13 inch oven-proof dish. Set aside.
To the cooled mushroom mixture, stir in the ricotta, 4 oz of cheese and egg. Fill the shells with spoonfuls of the spinach mixture and place in the marinara sauce open side up. Spoon the remaining sauce around the shells and drizzle a bit over the tops, but do not completely cover. Sprinkle the entire dish with the remaining cheese and panko.feel free to add more cheese if desired.
Cover with foil. Bake for 10 minutes at 400 degrees. Remove the foil and bake another 10-15 minutes until the sauce is bubbling and the cheese is golden brown and melted.