Preheat the oven to 180℃ fan/200℃/350F or Gas Mark 6 and line the loaf tin with baking paper, making sure the paper overhangs the edges of the tin so you can use this later to lift the roast out. Grease any parts of the tin not covered in paper with oil.
Cook the sweet potato slices in a small saucepan of boiling salted water for 7-8 minutes until softened but not breaking up. Drain and set aside.
Heat the oil in a medium saucepan over a low-medium heat, add the onion and cook for 5 minutes or until starting to soften, then add the sage and cook for a further 5 minutes until the onions are soft but not coloured. Remove the pan from the heat.
Mix the boiling water in a heatproof bowl with the stock cube until it has dissolved, add the breadcrumbs and stir well. Set aside for at least 5 minutes.
Reserve half of the sweet potato slices and set aside. Mash the remaining half well.
To the onion and sage, add the mixed nuts, tomato puree, Marmite, cranberry sauce, soaked breadcrumbs, mashed sweet potatoes and plenty of salt and pepper. Mix very well.
Sprinkle pistachios over the base of the tin. Add half the nut mixture and press down firmly. Arrange the remaining slices of sweet potato in a neat layer (you will need to overlap and/or trim them), then pile on the remaining nut mixture and press very firmly.
Bake in the oven for 40-50 minutes, or until the top is starting to turn dark brown and the loaf feels springy.
Remove from the oven and leave to stand for 10 minutes before turning out, using the baking paper to help you. Carefully cut into slices to serve.
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