Put the carrots, parsnips and swede in a pan of salted water set over a medium heat and bring to the boil. Reduce the heat to a simmer and cook for 8 mins until starting to soften, then add the sprouts and cook for 3-5 mins more until the carrots and parsnips are tender. Drain and transfer to a bowl of iced water. Leave to cool for 5 mins, then drain and leave to dry on a clean tea towel.

Meanwhile, roll the pastry out to a 1/2cm thickness, then cut out a 30cm circle. (You can use a large frying pan as a template, if needed.) Set aside in the fridge to chill.

Heat the oven to 200C/180C fan/gas 6. Heat the butter, vinegar and sugar in a 26-28cm ovenproof frying pan over a medium-low heat and, once the butter is sizzling, cook the red onion with the sage for 8-10 mins until starting to soften and brown. Remove from the heat and add the cooked veg to the pan in a layer on top of the red onions.

Lay the chilled pastry circle on top of the veg, tucking it in around the edge to enclose the veg. Bake for 30-40 mins until the pastry is golden and puffed up. Carefully invert onto a plate, then cool slightly before serving warm.