Mix the beef, 1 tsp mustard, garlic and egg yolk together with your hands. Season and form into 16 flat patties.
Sandwich two patties with approximately 1 tsp crumbled Gorgonzola and press the edges together to form a tight seal. Chill for 30 minutes or more.
Meanwhile, fry the onions in a little butter until soft. Stir in the sugar and balsamic vinegar and cook for a further 1-2 minutes until caramelised.
Cook the burgers on a hot barbecue, griddle or under the grill for 3-4 minutes a side until cooked through.
Mix the mayonnaise and remaining English mustard and spread a little on each burger bun. Finally, top with salad leaves, Gorgonzola burger and caramelised onions.
Cook’s Tip: Make sure that the patties are well sealed as the melted Gorgonzola will escape very easily!
Recipe courtesy of Gorgonzola