A few hours ahead, make the burger buns. I followed Mamen Lara's recipe on Cookpad called "Homemade Burger Buns"—they turn out amazing. I made 6 buns and froze the extras.

First, marinate the beef so it absorbs the flavors while you prepare the other ingredients. Place the ground beef in a bowl and add all the seasonings: salt, soy sauce, Worcestershire sauce, mustard, black pepper, and the two egg yolks. Cover with plastic wrap and set aside.

Start by making the caramelized onions for the Mediterranean burger. Heat a skillet with olive oil over low heat. Add the thinly sliced red onion and cook slowly until very soft and caramelized. Once done, add the balsamic vinegar and brown sugar, then set aside.

Now, cook the burgers. Divide the meat into 4 portions (about 5 1/3 oz or 150 grams each). Cook two at a time in a skillet with a little oil over high heat, about 4 minutes per side. After flipping the burgers, add cheddar cheese slices to two and brie cheese to the other two.