Preheat your oven to 375°F (190°C). Bring a large pot of salted water to a boil and cook the shells just until they’re pliable but still firm to the bite. Drain and set aside on a clean kitchen towel.
Make the filling: In a large bowl, whisk together pumpkin puree, ricotta, Gouda, egg, minced garlic, sage, nutmeg, lemon zest (if using), and a pinch of salt and pepper.
Stir in any greens you’re using. Taste the filling and adjust the seasoning with a touch more salt or pepper if needed.
Assemble the shells: Lightly oil a large baking dish. Spoon about 2 to 3 tablespoons of filling into each shell, then arrange them seam-side down in the dish.
Cover the bottom of the dish with a light layer of marinara. Spoon remaining sauce over and around the stuffed shells.
Bake, covered, for 25 minutes. Then uncover and bake an additional 8 to 12 minutes, or until the filling is set.
Rest briefly: Let the shells rest for 5 to 10 minutes after you remove them from the oven.
Final touch and serving: Sprinkle chopped parsley and a light dusting of extra grated Gouda on top.
Leftovers: Store any leftovers in an airtight container in the fridge for up to 3 days.
Notes
For a creamier filling, blend the mixture briefly.
If you like a saucier bake, add an extra cup of sauce.
For freezing, bake from frozen for about 45 minutes.