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Heat oven to 180C/160C fan/gas 4. Grease and line the base and sides of a 1lb (500g) loaf tin with baking parchment. Heat the oil in a large non-stick sauté pan and fry the onions for 10 mins until golden, stirring every now and then. Add the grated potato and garlic and cook for 5 mins more until the potatoes have softened.
Tip into a bowl, (reserve the pan) and add two-thirds of the chestnuts crushing them up as you go, then stir in the thyme, tamari and egg. Press into the loaf tin, roughly break the remaining nuts and arrange them on top then bake for 45 mins.
For the sauce mix the flour with a little of the bouillon to make a very wet paste. Return the pan to the heat and add all the ingredients, including the flour mix and stir over a high heat until well mixed and thickened. Keep simmering for a good 10 mins then taste as it should now be tangy rather than vinegary. If it is too sharp return to the heat and boil a little longer.
Carefully take the loaf from the tin and remove the paper. Scatter with thyme. If you're doing the Healthy Diet Plan, serve half in 2 thick slices with half the tomato sauce and chill the rest for another day, otherwise serve the whole recipe as a meal for four people.