Peel and finely chop the onion. Halve the squash and scoop out the seeds. Chop it into 2cm big chunks (you don't have to peel it, but you can if you prefer). Peel and chop the sweet potatoes to match the squash.
Warm a big pan over a medium heat for 2 mins. Add 1 tbsp oil and the onions. Season with a pinch of salt and pepper. Cook for 5 mins, stirring now and then, till it starts to soften.
While the onion cooks, halve the peppers. Scoop out the seeds and white bits. Roughly chop them. Peel and crush the garlic.
Stir the garlic into the onions with the cumin, cinnamon, ginger and chilli powder (use as much or as little chilli powder as you prefer. It is spicy). Cook and stir for 1 min, then add the squash, sweet potatoes and peppers.
Tip in the tin of tomatoes. Fill the tin with water (around 400ml) and add that to the pan, too. Drain the chickpeas and stir them in. Cover and simmer for 30 mins. Roughly chop the herbs and set to one side.
After 30 mins, take the lid off the pan and simmer for another 5-10 mins to thicken it up a little. Stir it every so often. Taste a little and add a pinch more salt and pepper if you think it needs it. Serve the tagine in bowls topped with chopped coriander leaves.