Start by heating the olive oil and butter in a large skillet over medium heat. Once the butter is melted and bubbling, add the diced sweet onion. Stir occasionally, allowing the onions to cook down slowly, taking about 15-20 minutes until golden brown.
While the onions are caramelizing, prepare the ground beef by combining it with 1 teaspoon of seasoned salt, garlic powder, and black pepper in a large bowl. Mix gently without overworking the meat.
Divide the beef mixture into four equal portions and shape into patties, making a small indent in the center of each to prevent puffing. Season both sides with the remaining seasoned salt.
Once the onions are caramelized, remove them from the skillet. Increase the heat to medium-high and cook the patties for about 4-5 minutes on one side, then flip and cook for an additional 3-4 minutes for medium doneness.
Just before the burgers are done, place a slice of provolone cheese on each patty and cover the skillet with a lid for about a minute to melt the cheese.
Assemble your burgers by spreading mayo on the bottom bun, adding the patty with melted cheese, and piling on the caramelized onions. Top with the other half of the bun.
