Onion Confit: in a sauce pan, melt the butter on low heat. Add onion slices. Lightly salt the onion to help them sweat. Stir often until dark brown but not too brown, around 30-40 minutes. Check often and add 1 tbsp water if the bottom of the pan is sticky. Remove from heat and set aside.

Sautéed Mushroom: bring a sauce pan on high heat with olive oil. When the oil is hot, add mushroom and salt. Cook it for 3-4 minutes until golden brown. Remove from heat. Sprinkle pepper on top and set aside.

Wagyu Patties: mix the meat and salt together and form two 150 g balls. Flatten it with a spatula. Heat up a frying pan on high heat. If your patties are highly marbled, no need to add fat. If your patties are leaner, add oil to the pan first and when it is hot, add butter. Fry one side of the patties on high heat until golden brown. Flip the other side on medium heat and cook until your desired doneness. Put the cheese slices on top of the patties until slightly melted.

Using the same frying pan, quickly toast bun halves on patties fat until light brown on the bottom.