Heat the oil in a large frying pan, and add the diced red onion. Cook over a medium heat for a few minutes, then add the minced garlic and grated carrot. Cook for a further 3 - 4 minutes. Finally, add the diced mushrooms and grated courgette, and cook for another 5 - 10 minutes, until all the vegetables are soft and any excess liquid has cooked off. When cooked, set aside to cool for at least 5 minutes.
Add the cooked chestnuts and drained chickpeas to a food processor, and blitz until coarsely chopped. Transfer the mixture to a large mixing bowl, and season generously. Add the cooled vegetables, oats and grated cheese, and mix to combine. Add the eggs, and mix well.
Thoroughly spray an 8 inch springform cake tin with oil, then add the chickpea roast mixture, and press it firmly into the corners, smoothing the top.
Bake at 190°C (170°C fan / Gas Mark 5) for around 1 hour 15 mins, or until the centre is firm and the crust is golden. Remove from the tin, and cut into slices to serve.