Preheat the oven to 180°C (350°F/Gas 4). Sauté the celery, onion and mushrooms in the oil over medium heat until they’re soft. Add the rest of the loaf ingredients and mix until combined. Turn into a 20 × 10 cm (8 × 4 inch) loaf (bar) tin, smooth the top, spread over the extra ketchup and cook for 1 hour.
The Mashed Potatoes
Cut the potatoes into pieces of the same size. Place them in a saucepan of cold salted water. Bring to the boil and cook for about 15 minutes until the potatoes are tender. Drain. Melt the butter in the milk in a saucepan over medium heat. Mash together the hot potatoes and the hot milk–butter mixture using a food processor, potato masher or a fork followed by a whisk. Season with salt and pepper. Add more milk if the mash is too dry. Serve hot.
In a saucepan, sauté the onion and mushrooms with 3 teaspoons of the butter until softened. Remove from the saucepan and set aside. Heat the remaining butter in the same saucepan. After a few minutes, when the butter starts to foam, add the flour. Stir constantly until the roux turns a light brown colour. Slowly pour in the beef stock, stirring constantly. When the mixture is smooth, increase the heat and bring to the boil. Add the mushrooms and onion and continue to cook, stirring, until it has the right consistency.