Heat your oven to 200°C/Fan 180°C/Gas 6. Tip the lentils into a bowl and cover them with cold water. Set aside to soak.
Halve the butternut squash and scoop out the seeds. Chop the squash into 1-2cm-wide chunks. You don’t need to peel the squash, as the skin is edible and will soften during cooking, but you can peel the skin off if you prefer. Spread the squash out on a baking tray. Drizzle with ½ tbsp oil and season with salt and pepper. Turn to coat in the oil and seasoning. Roast for 40 mins till the squash is tender and caramelised.
While the squash roasts, peel and finely chop the onion. Warm ½ tbsp oil in a medium pan over a medium heat for 2 mins. Add the onion with a pinch of salt and pepper. Cook, stirring now and then, for 5 mins till the onion is soft and glossy.
Roughly chop the tomatoes. Peel and grate the garlic and turmeric (the turmeric will stain things yellow, so grate it onto a plate and pop your grater straight into a sink of hot water).
Stir the tomatoes, garlic and turmeric into the onion with 1 tbsp Bombay spice mix. Drain the lentils and add them to the pan. Stir to coat in the spice mix.
Crumble in the stock cube. Add 600ml boiling water and stir to mix. Cover, bring to the boil and then reduce the heat a little so the daal is just simmering. Cook, stirring now and then, for 20 mins till the lentils are tender and the daal is thick. It should still be fairly soupy, so if it looks like it is drying out, turn the heat down and add a splash more water.
When the squash is roasted, add it to the daal. Fork in the spinach leaves. Cook and stir for 2 mins to just wilt the spinach.
Taste the daal and add a pinch more salt and pepper if you think it needs it. Ladle the daal into 2 warm bowls and serve.