Boil the shells: Bring a large pot of salted water to a rolling boil. Add the jumbo shells and cook for 2 minutes less than the packet suggests — they should be pliable but still quite firm to the bite. You'll notice the edges turning translucent and the pasta releasing a faint, wheaty aroma. Drain them in a colander, rinse with cold water to stop the cooking, and lay them out on a clean tea towel so they don't stick together.

Prepare the filling: Place the thawed, squeezed-dry spinach in a mixing bowl. It should feel almost crumbly in your hands, with no visible moisture. Add the ricotta, beaten egg, grated mozzarella, half the Parmesan, minced garlic, salt, pepper, and nutmeg. Stir with a wooden spoon until the mixture turns a uniform pale green and smells creamy and savoury. Taste a tiny bit — adjust salt if needed, but remember the cheeses will add more as it bakes.

Make the base sauce: Pour the passata and olive oil into a 20x30cm baking dish. Swirl the dish gently so the sauce covers the bottom evenly; it should look glossy and deep red. If you like a thicker sauce, stir in a tablespoon of tomato purée until it's fully incorporated.

Stuff the shells: Using a teaspoon, fill each shell with about a heaped tablespoon of the spinach-ricotta mixture. Don't overpack them — the filling should sit flush with the pasta edge, not bulge out. As you work, arrange the stuffed shells snugly in the dish, open side up, nestling them into the sauce. You should hear a soft squelch as they settle into the passata.

Top and bake: Sprinkle the remaining Parmesan and an extra handful of grated mozzarella over the shells. The topping should look like a light, snowy blanket. Bake in a preheated oven at 190°C (170°C fan) for 25–30 minutes. You'll know it's ready when the sauce is bubbling gently around the edges and the cheese on top is golden brown with darker patches — the smell of melted cheese and tomatoes will fill your kitchen.

Rest and serve: Remove the dish from the oven and let it sit for 5 minutes. This rest time allows the filling to set slightly, so the shells hold their shape when you lift them out. Garnish with fresh basil leaves if you like, then serve straight from the dish.