Preheat the oven to 190C (170C fan) / 375F.
Grease a large (3lb) loaf pan (about 24cm x 13cm x 7cm in size) and line with a strip of non-stick baking paper to cover the bottom and long sides. Leave a little excess paper hanging out to make it easy for you to lift out the loaf. (Alternatively you can use a non-stick loaf liner).
Pulse the cashews in a food processor until coarsely chopped, then place in a large mixing bowl. Process the bread into breadcrumbs, then add to the mixing bowl.
Place the lentils in a fine mesh sieve and wash under running water, agitating them with your hand until the water runs clear.
In a pan, bring the vegetable stock and lentils to the boil, then reduce the heat and gently simmer uncovered for 15 minutes, or until the lentils have become a soft purée and all the water has been absorbed. Stir regularly towards the end of the cooking time to prevent the lentils from sticking.
Meanwhile, heat the oil in a large non-stick frying pan (skillet) and cook the onion and celery until they start to soften. Add the garlic, mushrooms and sage, and cook for a further three minutes. Stir in the tomato purée and flour, then remove from the heat.
Add the contents of the frying pan to the mixing bowl with the cashews and breadcrumbs. Add the cooked lentils, parsley and salt, and stir until thoroughly combined.
Spoon the mixture into the prepared tin. Gently press down and level the top. Sprinkle with pine nuts and seeds, and press them into the loaf. Spread the cranberry sauce on top. Cover with foil and bake for 30 minutes. Remove the foil and bake for a further 30 minutes. When ready, the loaf should feel firm to the touch. Leave to cool in the tin for at least 20 minutes, before turning out and slicing (it will firm up more as it cools). If you’re making it ahead of time, you can leave it in the pan until completely cooled.
This loaf will keep for five days in the fridge, and for weeks in the freezer. If you’re not planning on eating the loaf in one go, you can slice it and freeze it in portions. The loaf can also be served cold and makes a great sandwich filling.