Roast the squash for deep flavour. Preheat your oven to 200°C (400°F). Toss the cubed squash with 2 tablespoons of olive oil, a pinch of salt, and a few cracks of black pepper. Spread them in a single layer on a baking tray. Roast for 25-30 minutes, flipping halfway, until the edges are caramelised and the cubes yield easily when pierced with a fork. The kitchen will smell sweet and toasty—that's your cue.

Soften the aromatics. While the squash roasts, heat the remaining tablespoon of olive oil in a large frying pan over a medium heat. Add the diced onion and cook, stirring often, for about 8 minutes until it turns translucent and soft, with a gentle golden edge. Stir in the garlic, dried sage, and nutmeg, and cook for another minute until the garlic is fragrant (you'll smell it immediately). Remove from the heat.

Cook the shells until al dente. Bring a large pot of salted water to a rolling boil. Add the jumbo shells and cook according to the package instructions, usually 10-12 minutes, but aim for just al dente—they'll finish cooking in the oven. Drain them carefully, rinse with cold water to stop the cooking, and lay them out on a clean tea towel, open side up, to cool slightly. They should feel pliable but not mushy.

Make the luscious filling. In a large mixing bowl, combine the roasted squash cubes, the cooked onion mixture, ricotta cheese, half the Parmesan, half the mozzarella, the toasted walnuts, and the beaten egg. Mash everything together with a fork or potato masher until the squash is mostly broken down but still has a few small chunks for texture. The mixture should be thick, creamy, and slightly chunky—taste and adjust salt and pepper.

Assemble with care. Spread a thin layer of passata (about 200 ml) across the bottom of a 23x33 cm baking dish. Using a small spoon or your fingers, fill each shell with a generous dollop of the squash mixture—don't overstuff, but make sure it's full. Place each filled shell, open side up, into the dish, nestling them close together. You should have a snug, crowded tray of shells.

Top and bake to golden perfection. Pour the remaining passata over and around the shells, spreading it gently with the back of a spoon. Sprinkle the remaining Parmesan and mozzarella over the top. Bake for 25-30 minutes, until the sauce is bubbling at the edges and the cheese is melted and golden brown in patches. The sound of a gently bubbling dish is the signal that it's almost ready. Let it rest for 5 minutes before serving—this allows the filling to set.

Garnish and serve. Scatter fresh sage leaves and a few extra chopped walnuts on top for a final touch of colour and crunch. Serve straight from the dish, spooning some of the tomato sauce over each portion.