Preheat the oven to 190°C (fan-assisted). While it heats, drizzle the olive oil into a large, ovenproof baking dish (roughly 30cm x 20cm) and set it aside. You want the oven fully up to temperature before the casserole goes in — this ensures the top browns evenly.

Sweat the aromatics: In a large frying pan over medium heat, add a splash of olive oil and cook the diced onion for about 5 minutes, until it turns translucent and releases a sweet, fragrant smell. Stir in the garlic and cook for another minute — the garlic should become aromatic but not brown, otherwise it will taste bitter.

Build the sauce: Pour in the tinned tomatoes and tomato purée, then add the oregano, basil, chilli flakes if using, and a generous pinch of salt and pepper. Stir well and bring to a gentle simmer. Let it bubble away for 8–10 minutes, stirring occasionally, until the sauce thickens slightly and turns a deep, rich red colour. You'll notice the raw tomato smell mellow into something savoury and inviting.

Prepare the ricotta mixture: In a small bowl, combine the ricotta cheese with the egg, half the grated Parmesan, and a pinch of salt. Whisk until smooth and creamy — the texture should be spreadable but not runny, like a thick yoghurt.

Layer the casserole: Spread a thin layer of the tomato sauce (about a third of it) across the bottom of the prepared baking dish. Arrange half the frozen ravioli in a single layer over the sauce — they can touch slightly but shouldn't overlap too much. Dollop half the ricotta mixture over the ravioli and spread it gently with the back of a spoon. Sprinkle a third of the grated mozzarella over the top. Repeat the layers: another third of the sauce, the remaining ravioli, the remaining ricotta, and another third of the mozzarella. Finish with the final third of sauce spread evenly across the top, then scatter the remaining mozzarella and the rest of the Parmesan over everything.

Bake until bubbling and golden: Place the dish in the preheated oven and bake for 30–35 minutes. You're looking for the sauce to be bubbling vigorously around the edges — you'll hear it hissing — and the cheese on top to turn a deep golden brown with darker patches. If the top isn't browning enough after 30 minutes, switch the oven to grill for the final 3–5 minutes, watching closely so it doesn't burn.

Rest before serving: Remove the casserole from the oven and let it sit on the counter for a full 10 minutes. This rest time allows the layers to set, so when you cut into it, the pieces hold their shape rather than turning into a soupy mess. Garnish with fresh basil leaves if desired, then serve straight from the dish.