Separate the baby gem leaves, wash well in cold water and leave them on the side.
Chop the red pepper into 3 long stripes, remove the seeds and the white bits from the inside. Chop the onion by first cutting off the two ends and peeling off the skin. Carefully chop the onion into half a cm thick discs.
Bring a pan to a medium-low heat and add a drizzle of olive oil. Fry the peppers slice and the onions discs carefully keeping the discs intact. Stir occasionally, keeping the heat on a medium/medium-low. Set aside both once are soft and caramelised.
Finely chop the garlic, sun-dried tomatoes and parsley. In a bowl add the minced beef, the garlic, sun-dried tomato, parsley, salt & pepper and the egg yolk. Mix well all together and make 3 equal balls of meat.
Cut the burger buns and toast the top and bottom part in a large frying pan on a medium-high heat. Set on the side. In the same pan, on a medium-high heat add a little touch of oil and start frying the meat balls. Leave them to fry for 30 seconds then with a masher, slowly press down each ball of meat to create a burger patty shape. Fry them for around 4 to 5 minutes each side for a medium cooked burger.
Only once the burger is almost cooked, add the sliced cheese on top of each patty. Add a tiny splash of water to the pan. Cover with a lid and let it melt for 30/40 seconds.
To build each burger, spread the aioli on the top and bottom part of the bun, then place in order to thebun base, the clean lettuce and red pepper, the patty with the cheese on top, the onion discs and close with the top bun slice. Gently press together and serve, Enjoy!