In a large skillet, heat the olive oil over medium heat. Add the thinly sliced onions and sprinkle them with a pinch of kosher salt to help draw out moisture. Cook the onions, stirring occasionally, for about 15-20 minutes or until they turn a rich golden brown and become sweetly caramelized. Once done, remove from heat and set aside.
Gently divide the ground beef into four equal portions, each weighing about 4 ounces. Form each portion into a loose ball without overworking the meat, which ensures a tender burger.
Heat a griddle or large skillet over high heat. Make sure it's very hot—this is crucial for achieving a beautiful sear on the burgers.
Place the beef balls onto the hot cooking surface, ensuring there is enough space between each one. Using a spatula or a burger press, firmly press each ball down to flatten it into a patty about 1/2 inch thick. Season the top sides with garlic powder, kosher salt, and freshly cracked black pepper.
Once the edges of the patties start to brown, which will take approximately 2-3 minutes, lay a slice of cheddar cheese atop each patty. Cover the skillet with a lid and cook for an additional minute, allowing the cheese to melt beautifully.
Meanwhile, in a separate pan over low heat, melt the butter. Slice each brioche bun in half and place the cut sides down into the buttery pan. Toast them gently until they are a delightful golden brown hue, taking care not to burn them.
On the bottom half of each toasted bun, place a cheesy burger patty. Generously spoon the caramelized onions on top, and then elevate your burger with any additional toppings you desire, such as lettuce, tomato, pickles, and your favorite condiments. Finish by capping it off with the top half of the bun.
Serve the Ultimate Smash Burgers immediately, while they're still hot and juicy. Pair them with a side of crispy fries or a refreshing salad for a complete meal.