7 ounces Gruyere, sliced4 brioche burger buns, halved and toasted
4 tbsp mayonnaise
1 tbsp balsamic vinegar
Place a large saucepan over low heat. Add oil and the onions. Cover with a lid and let cook for approximately 20 minutes or until soft but not colored, stirring often. Remove the lid then add the vinegar, sugar and salt and allow to cook for another 10 minutes or until caramelized and golden brown. Set aside to cool.
Place the ground beef in a large bowl then season and mix well. Form one-fourth of the mixture into a ball, then flatten. Place 1 tablespoon of the onions in the center of the patty, then encase with the surrounding beef to create a stuffed burger. Leave in the fridge for 1 hour to firm up.
Heat a frying pan over medium heat and add 1 tablespoon oil.
Cook burgers for 3 to 4 minutes on each side until cooked through. Place a few slices of Gruyere on each burger and place the lid on the pan to melt the cheese.
Chop the remaining caramelized onions and add to a bowl with the mayonnaise and balsamic vinegar and mix well to combine.
Spread the bottom of each bun with one-fourth of the mayonnaise mix and top with a burger. Serve right away.