Directions

1. Over medium heat, heat the oil in a frying pan. Add the onion and sauté until the onion is soft and translucent. Add the bacon and cook thoroughly. Add the garlic and thyme leaves and cook for 2 additional minutes. Remove from the heat and leave to cool for 15 minutes.
2. In a large bowl, mix together ground beef, the cooled onion mix, panko and egg yolk. Season with some salt and pepper.
3. In a separate bowl, mix together the Gruyere and havarti cheeses and then divide evenly into four piles.
4. Divide the beef mixture evenly into four sections and form into patties. Take one of the cheese piles and form into a ball. Press the cheese ball into the center of one of the beef patties and then bring the meat around the cheese to cover it. Lightly press down on the beef patty with the palm of your hand to flatten it. Repeat this process with each patty and cheese pile.
5. Place the beef patties on a plate and cover with plastic wrap and place in the refrigerator for at least an hour to chill.
6. In a food processor, combine the mayonnaise, mustard, parsley and basil and mix together until completely combined. Stir in the baby gherkins and put the mixture into a small bowl, cover and chill until ready to use.
7. When ready, grill the burgers on each side on low heat for approximately 3-4 minutes. Take the burgers off of the grill and wrap them individually in foil and place back on the grill for 4-6 minutes to melt the cheese center.
8. Grill the brioche buns, split side down, until toasted.
9. Spread the burger sauce on the bottom and top of each bun. On the bottom bun, place the lettuce leaf, followed by the tomato, burger and fried onions.