Preheat your oven to 200C/Fan 180C/Gas mark 6. Halve and scoop out the seeds of the butternut squash (no need to peel, but remove the skin if you prefer) and cut into small chunks. Tumble into a roasting tin and toss with a good pinch of salt and pepper and 1 tbsp olive oil. Slide into the oven and roast for 35 mins, till tender and caramelised.
While the squash is cooking, make the curry paste. Peel and finely chop or grate the garlic clove and ginger. Halve the chilli, scraping out the seeds and membrane for less heat, and finely chop. Separate the coriander leaves from the stalks (save the leaves for later) and finely chop the stalks. Tip all four ingredients into a bowl and stir in 2 tbsp of the tikka masala spice mix and 1 tbsp oil till everything is mixed into a paste.
Peel the onions and slice thinly. Chop the tomatoes roughly into small pieces.
Pour 1 tbsp oil into a large pan and bring to a high heat. Slide in the onions and cook for 2-3 mins, stirring frequently, till the are golden brown at the edges. Reduce the heat to medium and add the curry paste and a good pinch of salt and pepper. Cook for a further 2 mins.
Add the tomatoes and 100ml boiling water to the onions and spices and stir well, scraping up any bits caught on the bottom of the pan. Bring to a simmer, then reduce the heat to medium-low. Partly cover the pan with a lid and cook gently for 20 mins.
Meanwhile, rinse the rice under cold water and tip into a small pan. Add 300ml boiling water and a pinch of salt. Clamp on a lid, bring to the boil and then turn the heat right down. Gently cook for 8 mins, till all the water is absorbed and the rice is tender. Take the pan off the heat and let it sit, lid on, for 10 mins to finish cooking the rice.
When the curry sauce has had 20 mins, stir in the baby spinach leaves and the double cream, till the leaves are wilted. The squash should also be cooked by now. Fold the roasted chunks of squash through the sauce too, mashing the odd one up with the back of a spoon to thicken the sauce.
Pile mounds of fluffy rice onto plates and top with generous spoonfuls of the curry. Scatter a few coriander leaves over the top and serve.