Preheat the oven: Set your oven to 375°F (190°C). Lightly oil a 9x13-inch baking dish.

Cook the shells: Bring a large pot of salted water to a boil. Cook the jumbo shells until just shy of al dente, usually 1–2 minutes less than the box suggests. Drain and rinse briefly under cool water to stop cooking. Drizzle with a little olive oil and spread on a tray to prevent sticking.

Prepare the filling: In a large bowl, mix ricotta, 1 1/2 cups mozzarella, Parmesan, egg, garlic, herbs, Italian seasoning, and nutmeg if using. Add 1/2 teaspoon salt and a few grinds of pepper. Fold in the spinach if you’re using it. The mixture should be thick but creamy.

Taste and adjust: Cook a small teaspoon of the filling in a skillet or microwave for a few seconds to taste for salt and seasoning. Adjust as needed.

Layer the sauce: Spread 1 to 1 1/2 cups of marinara in the bottom of the baking dish. You want a thin, even layer to prevent sticking and keep the shells moist.

Fill the shells: Spoon about 2 tablespoons of filling into each shell. Nestle the filled shells open-side up into the sauced dish. Keep them snug but not crowded.

Top with sauce and cheese: Spoon the remaining marinara over the shells, leaving a few edges exposed. Sprinkle the remaining 1/2 cup mozzarella over the top, plus a little extra Parmesan if you like.

Bake covered, then uncovered: Cover the dish tightly with foil and bake for 20 minutes. Remove the foil and bake another 10–15 minutes, until the sauce is bubbling and the cheese is lightly golden.

Rest and finish: Let the shells rest for 5–10 minutes so they set and become easier to serve. Finish with a drizzle of olive oil and extra chopped basil or parsley.

Serve: Spoon two to three shells per person, with extra sauce from the pan. Add a simple salad and crusty bread if you’re making it a meal.