Preheat the oven to 180°C, fan 160°C, gas 4. Grease a 1.5-litre loaf tin and line the base with a strip of greaseproof paper. Heat the oil in a frying pan over a medium heat, add the onion, garlic and mushrooms and cook gently for 8-10 mins until softened.
Chop 150g of the pistachios and 150g of the walnuts, leaving the rest whole to decorate. Mix together the mushroom mixture, chopped nuts, breadcrumbs, seeds, thyme, egg, soy sauce and cheese until combined. Season.
Tip into the loaf tin, pressing down firmly. Roast for 35 mins, or until the loaf is set. Rest for 10 mins while you make the gravy.
In a medium pan, melt the butter and stir in the flour. Cook over a medium heat for 2-3 mins until the mixture is thick and smells biscuity. Stir in the yeast extract and redcurant jelly. Remove from the heat and stir in the vegetable stock. Return the pan to the hob and bring to a simmer. Cook for 5 mins until you have a smooth and velvety gravy. Season with salt and freshly ground black pepper.
Turn the nut loaf out onto a serving plate. Toss the whole nuts in a frying pan over a low heat for 1-2 mins then stir in the butter until melted. Scatter the nuts over the top of the loaf, and garnish with parsley. Serve sliced with the gravy and your choice of roasted veg.