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Put a large saucepan of water on to boil. Quarter the squash with a large, heavy knife, then peel each wedge and seed with a sharp spoon. Cut both the squash and the sweet potatoes into large chunks, about the size of toasted marshmallows. (If the sweet potatoes are long and thin, diagonal chunks look better.) Throw the chunks into the boiling water, cover the pan and bring back to the boil. Remove the lid and cook for 5 minutes or until slightly tender, then drain and let cool.
Cut each onion into 6 chunky wedges, leaving a little bit of the root attached so the layers don’t fall apart. Put a large, non-stick frying pan, about 30cm/12in from edge to edge, over a medium-high flame. (You need a pan with an ovenproof handle, and we’ve an offer on one, below right). Heat the butter and oil, then tip in the onions and fry for 4-5 minutes, stirring often, until well-coloured.
Keeping the fat at a lively simmer, add the garlic and fry for 1 minute, stirring almost constantly so it colours but doesn’t burn. Stir in the coriander, cumin and chilli powder and fry for a scant minute, stirring constantly. Fry the tomatoes, cut side down in the pan, for 1 minute.
Remove the pan from the heat, then gently stir in the kalonji seeds. Add the squash and sweet potatoes and, using 2 spatulas or wooden spoons, carefully toss the chunks until well coated in the syrupy, spicy fat. Season, then leave to cool.
Roll out the pastry so that the shorter sides are 30cm/12in. Lay it over the vegetables, then cut the pastry tightly around the edge of the frying pan with a small knife or scissors, discarding the trimmings (or using them for another recipe). Refrigerate the entire ensemble for at least 30 minutes (or see Getting Ahead, below).
Preheat your oven to 220C/Gas 7/fan 200C. Brush the pastry with the egg wash, then slide the frying pan on to the middle shelf of the oven. Cook for about 25 minutes, checking halfway through that the pastry isn’t browning too quickly: if it is, reduce the heat by 10 degrees. Remove when the juices are bubbling around the edges and the pastry is dark golden-brown.
Use a thick oven cloth to remove the pan from the oven and leave it wrapped firmly around the handle to avoid burns. To serve the tart, invert a large, flat plate over the pan and tip the tart out on to it – or else, serve it straight from the pan.