1. Make the Sauce In a medium mixing bowl, combine the mayonnaise, ketchup, mustard, ¼ cup of caramelized onions (prepared in the next step), finely chopped pickles, salt, pepper, sugar, and vinegar. Stir everything together thoroughly until fully combined and smooth in texture. Cover the bowl tightly and refrigerate the sauce while you prepare the remaining components — chilling allows the flavors to meld together and intensify by the time you’re ready to assemble the burgers.

2. Caramelize the Onions Place a small skillet or saucepan over low heat and add the olive oil along with the butter, letting it melt together. Add the diced white onions to the pan, stirring to coat them in the fat. Cook the onions slowly over low heat for 15 to 20 minutes, stirring occasionally to prevent sticking or burning, until they turn soft, deeply golden brown, and fully caramelized. Once done, remove the pan from the heat and set the caramelized onions aside, reserving a portion for the sauce and the rest for topping the finished burgers.

3. Shape the Burger Patties Divide the ground beef into 4 equal portions, each weighing approximately 100 grams. Working with one portion at a time, roll the beef into a loose ball between your palms, then flatten it out into a thin patty roughly ¼-inch thick, shaping it into a rough oval or round similar in size to your croissant halves. Since the patties will shrink somewhat as they cook and release fat, make each one slightly wider than the bun you plan to use. Season one side of each patty generously with salt and pepper, then place the shaped patties on a plate or tray and keep them chilled in the refrigerator until you’re ready to cook.

4. Make the Garlic Butter In a small bowl, combine the melted unsalted butter with the chopped parsley, garlic powder, garlic paste, and a pinch of salt to taste. Stir together until fully combined and the garlic and herbs are evenly distributed throughout the butter.

5. Toast the Croissant Buns Slice each croissant in half horizontally, creating a top and bottom piece. Brush the cut sides of each croissant half generously with the prepared garlic butter. Heat a dry skillet or griddle over medium heat, then place the croissant halves cut-side down onto the hot surface. Toast for a minute or two, watching closely, until the cut sides turn golden brown and slightly crisp. Remove the toasted croissants from the skillet and set them aside.

6. Cook the Burger Patties Heat the same skillet or griddle over medium-high heat and add a thin layer of oil to the surface. Working in batches of two patties at a time to avoid overcrowding the pan, add the seasoned beef patties to the hot skillet. Let them cook undisturbed for about 2 minutes on the first side, until a deep brown, crispy crust forms on the bottom. At this point, add a small amount of mustard to the top of each patty and spread it evenly across the surface using the back of a spoon. Flip each patty over, and immediately place a slice of American cheese or sharp cheddar on top of each one. Continue cooking for another 1 to 2 minutes, until the cheese has fully melted and the patty is cooked through to your desired doneness.

7. Assemble the Burgers Spread a generous layer of the prepared sauce onto the bottom half of each toasted croissant. Place a cheesy beef patty on top of the sauce, followed by a spoonful of the reserved caramelized onions. Add lettuce and sliced tomato on top of the onions, along with any additional toppings you’d like. Finish by placing the top half of the croissant over everything, pressing down gently to help the burger hold together. Serve immediately while everything is still warm.