1. In a cast iron pan over medium heat, add oil and butter. Once butter has melted, add sliced onions and about a teaspoon of Weber seasoning. Mix and let the onions caramelize for about 10-15 minutes.
2. Once the onions are mostly brown/caramelized, push them to one side of the pan. Season both sides of your patties and place down in the open side of the pan. Cook for about 2-3 mins on the first side and after the flip, add the cheese and let it melt while the rest of the burger cooks.
3. After all burgers are cooked, remove them from the pan with the onions.
4. Add apple cider vinegar to deglaze the pan and scrape the bottom to remove any bits of seasoning. Add the apples and cook gently for about 3 minutes then remove from pan.