For the sauce, stir all of the ingredients, including some salt and pepper, together in a medium pan. Add 100ml/3½fl oz water and bring to the boil. Reduce the heat and simmer for about 10 minutes, stirring occasionally, until the cranberries have burst and softened and the apples and dates have cooked down to give a chunky sauce.
Remove from the heat and serve warm or cold. This will keep in an airtight container in the fridge for up to a month.
For the pastry, place all the ingredients in a food processor and whizz them together until they start to form a smooth ball of dough. If using plain spelt flour, add 3 tablespoons water to help bring it together. If using wholemeal flour, you will need to add a further 1-2 tablespoons water. Alternatively, to make by hand, rub the flour into butter until the mixture resembles breadcrumbs. Lightly beat the egg in a small bowl and stir into the crumbs really well using a table knife until lumps start to form. Add the water (as above) to bring it together.
Wrap the pastry in cling film and chill in the fridge for 30 minutes.
For the mini Wellingtons, heat the oil in a large frying pan over a low-medium heat. Add the squash and/or sweet potato, onion and thyme leaves and cook for about 15 minutes, stirring from time to time, until the squash is tender.
While this is cooking, lightly dust a work surface with flour and roll out the pastry to a 36cm/14in square, roughly 1mm thick (or as thin as you can get it). Make sure the edges are straight and neat, trimming them with a knife if necessary. Cut this square into quarters and arrange the 4 squares on a large baking tray lined with baking parchment (or double them up with parchment between each pair if the tray isn’t large enough). Cover with cling film and refrigerate for about 10 minutes.
Once the vegetables are cooked, add the garlic and cook for a minute more, then tip the mixture into a large bowl. Return the pan to the heat, add half the spinach along with 1 tablespoon water. Cook for 1-2 minutes while stirring, until just wilted. Tip into a colander in the sink, then repeat with the remaining half of spinach. Rinse the whole lot under cold water until cool enough to handle, then picking up handfuls at a time, squeeze as much water out of the spinach as possible and add it to the vegetable mixture.
Add the lentils, chestnuts, pine nuts, apple, nutmeg and cinnamon. Add the paprika if you fancy it – I love paprika in pretty much everything. Stir together well and then season, to taste, with salt and pepper
Divide the filling mixture evenly among the 4 squares of pastry, spooning it into the middle of each. Allow a border of about 2cm/¾in around the mixture and brush with beaten egg (reserve the remaining egg). Fold 2 opposite corners of the pastry inwards to meet each other in the middle. Then fold the other opposite corners into the centre also, but just overlapping the first bit of pastry so they stick securely to form a pouch. Flip them over so the sealed side is down and use a small, sharp knife to slash the tops with 3 ‘cuts’. Lay them out in a single layer on a baking tray and pop them back in the fridge for 10 minutes to firm up.
Preheat the oven to 200C/180C Fan/Gas 6.
Remove the Wellingtons from the fridge and brush the tops and sides with the remaining egg. Bake for 25-30 minutes or until the pastry is cooked and golden-brown and the filling is piping hot. Remove from the oven and serve with salad leaves.