Preheat the grill to high setting.
First start by preparing the caramelized red onions. In a pan heat up 2 tbsp olive oil and add the sliced red onions along with a good pinch of Maldon salt. Turn the heat down low and allow the onions to slowly cook down for 10 minutes until soft.
While the onions are cooking, place the two portobello mushrooms onto a foil lined baking tray. Remove the stem and then stuff the mushroom with the crumbled stilton cheese. Season with some cracked black pepper and then place them under the grill for 5 minutes -until the mushroom is tender and the cheese is melted and bubbling.
Going back to the onions, after they have softened for 10 minutes add the balsamic vinegar and caster sugar and cook gently for another 5 minutes until sticky and caramelized. Remove from the heat.
Now you can assemble your burgers. Start with the toasted brioche bun base, spread a thin layer of Dijon mustard on, if desired. Next top with the stilton filled portobello mushroom, season with some Maldon salt and then pile on some of the caramelized onions. Finally top with the wild rocket and the brioche burger lid. Serve with some more Dijon mustard, if you’d like.