Heat the oven to 180°C (350°F, gas mark 4). Lightly grease a 900 g (2 lb) loaf tin.
Heat the oil in a large saucepan over a medium heat and gently fry the garlic, mushrooms and onions for 7-8 minutes until they are tender and lightly browned, stirring frequently.
Add the brandy to the pan and allow it to simmer for 1-2 minutes until reduced, then remove the pan from the heat and leave the mixture to cool for about 3 minutes.
Break the chestnuts into pieces and stir them into the mushroom mixture with the egg, breadcrumbs, chestnut purée, orange zest and juice, parsley, thyme and salt and pepper to taste, using a wooden spoon to break up the chestnut purée.
When the mixture is thoroughly combined, spoon it into the loaf tin, smooth over the top and bake it for 45 minutes, or until the top is browned.
Leave the terrine to cool in the tin, then turn it out onto a plate and cut it into neat slices. Sprinkle a little chopped coriander or basil over the top and serve it with mixed salad leaves.
Chestnuts not only have a great flavour, they have just a fraction of the fat of most other nuts.