1. To make the agrodolce onions, heat the oil, butter and onions in a frying pan over a medium heat. Once sizzling, reduce the heat to low and fry for 20 minutes, stirring frequently, until the onions have softened.
2. Tip the onions into a bowl. Remove the frying pan from the heat and add the balsamic vinegar, sugar and 1 tablespoon of boiling water. Stir to dissolve the sugar, then mix with the onions and leave to cool.
3. To make the burgers, tip the meat into a large bowl and, using clean hands, break it up. Add the rest of the burger ingredients and gently mix to combine. Divide into 8, form into balls and then flatten each ball into a burger shape, about 3cm thick. Put on plates, then cover and chill for 30 minutes or until needed.
4. Before you start barbecuing, cut the Taleggio into 8 slices lengthways. Halve the burger buns and put the slices of pancetta on a plate.
5. Barbecue the pancetta until crisp, then keep it warm in a low oven. Cook the burgers for 4-6 minutes on each side, until cooked throughout. Top each burger with a slice of Taleggio and leave on the grill for a minute or so to melt the cheese. Add the buns, cut-sides down, to lightly toast them.
6. To assemble the burgers, spread the bottom bun halves with mayonnaise. Add the rocket, and the melting Taleggio burgers, cheese-side down, then spoon over the agrodolce onions and crisp pancetta. Top with the bun lids and serve straight away.