Make the filling: using a large skillet, heat the olive oil and saute the chopped mushrooms until almost all the liquid has evaporated.
Mix in the spinach leaves and white wine. Let the mixture cook until the leaves are wilted and most of the liquid has evaporated. Transfer to a bowl.
Make the sauce: melt the butter in the same skillet and saute the onions and garlic until translucent. Stir in the marinara sauce, Italian seasoning, salt, and pepper. Simmer for about 10 minutes.
Cook the shells until al dente, drain, and drizzle with a little olive oil to prevent them from sticking. Do not overcook them.
Add the ricotta cheese, chopped fresh oregano, and one cup of grated Parmesan to the bowl with the spinach and mushrooms. Mix well and season with salt and pepper.
Stuff the shells: stuff each cooked shell with 1 ½ - 2 tbsp of filling.
Place the shells on top of the sauce in a circular pattern, and sprinkle the remaining Parmesan cheese evenly on top of the shells.
Place the lid on and simmer for about 7 minutes or until everything is heated through and the cheese is melted.