Heat your oven to 200°C/Fan 180°C/Gas 6. Trim the stalk off the squash. Slice the squash in half and use a spoon to scoop out the seeds (no need to peel, although you can if you prefer). Slice the flesh into thick chunks and scatter into a medium ovenproof dish. Toss with 1 tbsp olive oil and a pinch of salt and pepper till evenly coated, then slide into the oven and bake for 20 mins.
Meanwhile, peel the red onions and thinly slice them. Peel the garlic cloves and crush or finely chop them. Slice the chestnut mushrooms.
Pour 1 tbsp olive oil into a medium pan and bring to a medium heat. Slide in the chopped onions and mushrooms. Sprinkle in a pinch of salt and pepper and fry for 5 mins, stirring occasionally, till glossy and softened.
Stir the chopped garlic into the pan. Spoon in 1 tsp dried oregano and cook for a further 1 min.
Drain and rinse the chickpeas. Tip them into the pan. Pour in the cherry tomato passata. Stir well and bring to a simmer, then cook for 5 mins. Tip the spinach leaves and half the pot of mascarpone into the sauce and stir till the leaves are wilted and everything is combined. Taste the sauce and add a little more salt and pepper if needed.
The squash should be roasted by now. Pour the sauce into the dish with the squash and gently turn to mix. Flatten out evenly with the back of a spoon. Sprinkle the breadcrumbs over the top.
Slide the dish back into the oven and roast for 20 mins, till golden brown and bubbling. Spoon onto a couple of warm plates and serve.