Halve the squash, scoop out the seeds, then peel it. Chop into bitesize chunks. Trim the courgettes and chop into chunks. Slice the mushrooms. Peel and thinly slice the onions.
Peel the garlic and ginger and tip into a food processor. Separate the chervil leaves from the stalks (save the leaves for later). Roughly chop the stalks. Trim the top off the chilli. Add the chilli and chervil stalks to the processor along with the Thai curry paste and 1 tbsp oil. Whizz till finely chopped and well combined. No food processor? Simply finely chop everything as finely as possible.
Pour 1 tbsp oil into a large pan and bring to a medium-high heat. Slide in the onions and fry for 5 mins, stirring occasionally, till softened and brown at the edges. Add the mushrooms and chopped squash and the curry paste. Cook for a further 3-4 mins, giving everything a good mix every now and then.
Crumble in the coconut milk and pour in 1ltr boiling water. Crumble in the stock cube and stir well. Return the water to the boil and then reduce to a simmer. Partly cover with a lid and cook for 20 mins.
Once the curry has had 20 mins, stir in the courgettes and cook for a further 15-20 mins, till everything is tender. If you like your curry a little thicker, use a stick blender to blend some of the vegetables. Alternatively, use a masher to crush the vegetables.
Have a taste of the stew and add more salt and pepper if needed. Ladle the curry into deep bowls and top with the reserved chervil leaves.