Smash Burger with Caramelised Onions on Brioche Buns

How To Make 

1. For the Caramelized Onions:
   Heat a large pan over medium-low heat, add the butter and olive oil, and let it melt.
2. Add the thinly sliced onions to the skillet and stir to coat them with the butter and oil.
3. Season the onions with a pinch of salt and a little sugar to help with caramelisation.
4. Cook the onions on low heat, stirring occasionally, for about 30-40 minutes or until they are soft, brown, and caramelised.
5. For the Smash Burger Patties:
   Divide the beef mice into 4 equal portions and shape them into balls.
6. Heat a cast-iron pan over high heat until it’s very hot. You want it smoking hot for the best sear.
7. Place the balls of meat onto the hot skillet and press down with a heavy spatula or a burger press to flatten them into thin patties.
8. Press down firmly to get that classic crispy crust.
9. Season the patties with salt and freshly ground black pepper.
10. Cook for about 1-2 minutes on each side, or until the edges are crispy and the centre is cooked to your desired level of doneness.
11. Place a slice of cheese on each patty while they are still in the pan. Cover with a lid to help the cheese melt. Once the cheese is melted, remove the patties from the heat.
12. To assemble the Burgers
    Slice the brioche buns in half and lightly toast them in butter for a minute or until they are golden brown.
13. Spread a thin layer of mayonnaise on the bottom bun and add mustard and ketchup to taste.
14. Place a patty with the melted cheese on top of the sauce.
15. Add a generous portion of caramelized onions on top of the patty.
16. If desired, add pickles, lettuce, and tomato slices.
17. Place the top bun on the assembled ingredients.