In a large pot, bring salted water to a rolling boil. Carefully add the jumbo pasta shells and cook according to package instructions until they are al dente. Once cooked, drain and set the shells aside to cool.
On medium heat, warm the olive oil in a large skillet. Add the finely chopped onion and sauté for about 3-4 minutes, or until it turns a translucent golden color.
Next, add the minced garlic followed by the diced mushrooms to the skillet. Sauté for about 5-7 minutes, stirring occasionally, until the mushrooms are tender and most of their moisture has evaporated.
Stir in the chopped spinach, cooking just until it wilts, which should take about 2 minutes. Once done, remove the skillet from heat.
In a separate large bowl, mix together the ricotta cheese, half of the shredded mozzarella, and half of the grated Parmesan. Add the sautéed spinach and mushroom mixture along with the oregano, basil, salt, and pepper. Mix thoroughly until all ingredients are well combined.
Pour a thin layer of marinara sauce into the bottom of a 9x13 inch baking dish.
Carefully fill each cooled jumbo shell with the spinach and mushroom mixture, and place them in the baking dish with the open side facing upward.
Once all shells are neatly arranged in the dish, gently pour the remaining marinara sauce over them. Finish by sprinkling the remaining mozzarella and Parmesan cheese on top.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and continue baking for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Allow the dish to cool for a few minutes before serving.
Notes
Serve in individual bowls, garnished with fresh basil and Parmesan cheese.