Depending on how thick your burgers are, you might find the middle of them is quite squidgy and raw, which is fine if you are OK with raw egg. One option is to pre-bake them in the oven on a low heat for 15 minutes before frying them to finish them off. This is also a good idea if you are barbecueing them. The oven baking firms them up so they don't collapse on the barbecue.