01 -
Set oven temperature to 375°F (190°C) and allow to fully preheat.
02 -
Cook large pasta shells in a large pot of generously salted boiling water until just al dente. Drain thoroughly and gently toss with a drizzle of olive oil to prevent sticking. Set aside.
03 -
Heat olive oil in a large skillet over medium heat. Add the finely chopped onion and cook until softened, about 3 minutes. Add minced garlic and cook for 30 seconds, stirring constantly. Incorporate chopped mushrooms and thyme; sauté until the mushrooms release their liquid and turn golden brown, about 8 minutes. Season with salt and pepper. Remove from heat and allow mixture to cool slightly.
04 -
In a large mixing bowl, combine ricotta, softened cream cheese, grated Parmesan, shredded mozzarella, chopped parsley, and the cooled mushroom mixture. Mix thoroughly. Adjust seasoning with salt and pepper to taste.
05 -
In a separate bowl, mix tomato passata or marinara sauce with dried oregano, sugar (if using), and season with salt and black pepper.
06 -
Spread half of the prepared tomato sauce evenly over the bottom of a 9x13-inch (23x33 cm) baking dish.
07 -
Fill each cooked pasta shell generously with the cheese and mushroom mixture. Arrange the filled shells, open side up, in the prepared baking dish atop the sauce.
08 -
Pour the remaining tomato sauce evenly over the stuffed shells. Sprinkle with the mozzarella and grated Parmesan reserved for topping.
09 -
Cover the baking dish tightly with foil and bake for 20 minutes. Remove foil and continue baking for an additional 15 minutes, or until the cheeses are golden and the sauce is bubbling.
10 -
Allow the baked shells to rest for 5 minutes before serving. Garnish with additional parsley if desired.