Preheat the oven to 200°C (180°C fan).

Heat 2 tablespoons of olive oil in a large ovenproof pan or shallow casserole dish over a medium heat. If you don't have an ovenproof pan, use a large frying pan and move everything into a baking dish later. Add the onion and cook for 8–10 minutes, stirring now and then, until soft, sweet and just starting to turn golden.

Meanwhile, cook the pasta in a large pan of well-salted water as per packet instructions. Scoop out a mug of the starchy pasta water before draining.

Stir the garlic, tomato purée and chilli flakes (if using) into the onion. Cook for 1–2 minutes until fragrant and the purée darkens to a deep brick red. If it starts to catch, add a splash of water.

Pour in the passata, tear in most of the basil and season with a good pinch of salt and pepper. Simmer for 7–8 minutes, stirring occasionally, until slightly thickened.

Tip in the butter beans with all their bean stock, then add the pasta and the spinach (if using). Fold together and let it bubble for 5–6 minutes until the spinach wilts and the sauce is glossy and thick. Loosen with pasta water if needed. It should look a little saucier than you'd serve it, as the pasta keeps soaking up sauce in the oven. For extra creaminess, crush a few beans against the side of the pan, keeping most of them whole for bite. Taste and adjust the seasoning.

If you used a frying pan, spoon everything into a baking dish. Scatter over the cheddar, tear the mozzarella over the top and finish with the parmesan.

Bake for 8–10 minutes until melted and bubbling at the edges. Rest for 5 minutes, scatter over the remaining basil and serve with garlic bread and a crisp green salad for freshness.