You have to work the meat after it’s been out of the frig for a bit. Get it softer and easier to form. The patties get pressed out pretty thing, my guess is about 6-7” across. On parchment or wax paper you press them out. Place the filling in the center leaving about a half inch around the entire patty untouched by filling. I recommend using diced bacon and diced onions to make it easier. The onions i made were rather large and made it a bitch to seal them up. I also recommend doing the fillings the day before because once they are cool they stay better. It also keeps the juices from the onion and mushrooms from not sealing the patties.
After you lay the second patties on top, I press down the entire edge semi flat. Then I carefully lift them and pinch it together all the way around. Then I tap the edge all the way around to round it back up. If you see a seam between the patties it will separate when cooking which will destroy the burger. If you notice a seam pinch it until you don’t see it anymore then pat it back to round.
These burgers are not very easy the first couple of times. As you do them you learn how to prevent disasters. None of the 8 burgers I made separated this time. I usually have one and it becomes mine because i won’t serve it to anyone.
The other key is putting them back in the frig for atleast an hour covered in plastic wrap on a baking sheet. This will make them stiff and the seal holds better.
Another tip, if doing them on the grill no direct heat, you will lose all the juices due to the extreme temperature. These cook up much better in a pan then a grill in my opinion but i had too many to cook for that. I set the charcoal on one side of the grill and the burgers on the other. I flip them in place. I’m sure I’m forgetting something but this is a good start.