Preheat oven to 375°F (190°C). Cook the jumbo shells in a large pot of boiling salted water according to package directions, aiming for al dente. Drain carefully, rinse with cool water to stop the cooking, and set aside.

While the pasta cooks, heat olive oil in a large skillet over medium heat. Add the chopped onion and cook for 5-7 minutes until softened. Add the minced garlic and cook for 1 minute more until fragrant.

Add the finely chopped mushrooms and walnuts to the skillet. Cook, stirring occasionally, for 8-10 minutes until the mushrooms have released their liquid and the mixture is browned and resembles ground meat. Season with a pinch of salt and pepper and set aside to cool slightly.

In a large bowl, combine the whole milk ricotta, 1 cup of shredded mozzarella, grated Parmesan, the egg, nutritional yeast, and dried oregano. Mix well. Stir in the completely dry spinach and the cooled mushroom-walnut mixture. Taste and adjust seasoning with salt and pepper.

Spread about half of the marinara sauce evenly across the bottom of a 9x13 inch baking dish. Using a small spoon, generously fill each cooked pasta shell with the ricotta mixture.

Arrange the filled shells snugly in a single layer in the baking dish. Pour the remaining marinara sauce over and around the shells. Sprinkle the remaining 1 cup of mozzarella and a little extra Parmesan cheese over the top.

Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for another 10-15 minutes, until the sauce is bubbly and the cheese is melted and golden brown.

Let the stuffed shells rest for 5-10 minutes before serving. Garnish with fresh chopped parsley or basil.