Bring a large pot (4qt) of lightly salted water to a boil over high heat. Cook the pasta according to package directions for 8 minutes, or al dente. Drain. DO NOT RINSE SHELLS.

Place and separate shells open-side down on plastic cutting sheet to dry. Once dry, spray baking sheets with oil and place them in right side up.

Heat 1 tsp of olive oil in a large saucepan over medium heat. Add chopped onion, garlic and chorizo and cook until browned, about 10 minutes.

Add full jar of tomato sauce, season with sugar, salt, and freshly ground black pepper, and simmer until sauce is slightly thickened, at least 10 minutes. Set aside to cool slightly.

Combine remaining ingredients--mushroom, spinach, ricotta, mozzarella, ONE cup/half of the parmesan, lightly beaten egg, basil, pinch of: nutmeg, ground oregano and parsley in a large bowl and mix to evenly combine.

Spoon half of the prepared meat sauce in mixture. Stuff each pasta shell with a heaping tablespoon of the filling and arrange the stuffed shells in the dish. Spoon remaining sauce over the shells.

Top with remaining parmesan and mozzarella cheese. Cover with foil. Heat the oven to 375°F and arrange a rack in the middle. Bake until pasta is cooked through and sauce begins to bubble, about 25 minutes.

Remove foil. Broil for 2-5 minutes.