Step 1: Cook the Pasta

1. In a large pot of salted boiling water, cook the jumbo shells according to package instructions until al dente. Drain and set aside to cool slightly.

Step 2: Prepare the Filling

1. In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for 3-4 minutes until softened.
2. Add the garlic and sliced mushrooms, cooking for an additional 5-7 minutes until the mushrooms are browned and any liquid has evaporated.
3. Stir in the fresh spinach and cook until wilted (if using frozen spinach, ensure it is well-drained). Remove from heat and let cool slightly.
4. In a large bowl, combine the sautéed mushroom and spinach mixture with ricotta cheese, half of the mozzarella cheese, Parmesan cheese, Italian herbs, salt, and pepper. Mix until well combined.

Step 3: Assemble the Dish

1. Preheat your oven to 375°F (190°C).
2. In a medium bowl, mix the marinara sauce with heavy cream (if using) to create a creamy sauce.
3. Spread a thin layer of the marinara sauce on the bottom of a 9×13 inch baking dish. Using a spoon, fill each cooked pasta shell with the mushroom and spinach filling and place them in the baking dish, open side up.
4. Pour the remaining marinara sauce over the stuffed shells, ensuring they are well covered. Sprinkle the remaining mozzarella cheese on top.

Step 4: Bake

1. Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
2. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.

Step 5: Serve

1. Once baked, remove from the oven and let it cool for a few minutes. Garnish with fresh parsley before serving.
2. Serve warm and enjoy this creamy, comforting dish!