Finely dice the onion and stir fry it until it slightly turns brown. Set aside and leave it to cool.
Mix the beef, lamb mince and salt and keep mixing it until it becomes sticky.
Mix the milk and breadcrumb separately in another bowl.
Add the milk and breadcrumb mixture, egg, pepper, Worchester sauce, mustard and onion with the meat mince. Mix them well.
Divide the mixture into four portion equally.
Take one portion and pat it between two hands 10 times. Put one(or two!) dice of mozzarella in the center and cover it. Flatten and mould it into rounded shape.
Add the oil on the grill pan, grill it on both sides to seal it.
Preheat the oven and bake it for 180 C for 6-8 minutes depending on how you like them. Alternatively, u may turn to medium heat and leave it on the grill pan and grill it for 6-7 minutes for each side. Again, depending on how you like them!
Add any veggies toppings and sauces that u may like with the burger bun! I personally like to add a sunny side up on top of the burger meat! The egg yolk would melt on the burger meat, Gorgeous!