Heat the butter and oil in a large, ovenproof frying pan over a medium-high heat. Stir in the onions with a large pinch of salt and cover with 500ml boiling water. Let it bubble away for 15-20 mins or until all the water has evaporated.

Heat the oven to 220C/200C fan/gas 7. Stir the sugar and wholegrain mustard into the onions and transfer to a bowl. Brown the sausages all over lightly in the pan. Return the onions to the pan and nestle the sausages on the top. Pour over the ale, bring to a simmer then transfer to the oven. Cook for 45-50 mins, turning the sausages a few times, until they turn dark brown and the liquid has reduced.

Meanwhile, combine the Dijon mustard and mayonnaise and season to taste. Slice open the hot dog buns. Serve with a slather of dijonnaise down the centre of the bun and top with the beer-glazed onions, sausages and a sprinkle of crispy onions, if you like.