In a pan over low heat, melt the butter and cook the chopped onion until golden brown. Remove from heat and set aside to cool completely.
In a small bowl, combine the panko and milk. Mix well until it forms a paste.
In a large bowl, combine the minced beef, minced pork, cooled onion, panko paste, eggs, and all the seasoning.
Mix well by hand until the mixture turns pale in colour and becomes sticky.
Divide the mixture into 8 equal portions.
Toss each patty back and forth between your hands about 20 times to release air pockets. This creates a denser and juicier patty.
Refrigerate until ready to cook. Remove from the fridge at least 30 minutes before cooking to allow the patties to come to room temperature.