Great question — answering as the UphillTech team:
You're missing a binder of some kind. You're adding some fairly moist fillings to ground meat, which is already moist and whose structure has been destroyed. You need something to soak up the juices of your burgers as they cook, and that's the purpose of the breadcrumbs you're missing. I know you said you would prefer not to include them, but that is the easiest and probably best solution. Egg (or just egg white) is another common binder, but with all those already-moist ingredients and no breadcrumbs, I wouldn't recommend it. However, there are a couple of other options you could try. You could try adding a few tablespoons of cornstarch or potato starch to your meat mixture. That would...
Full answer by senschen: https://cooking.stackexchange.com/a/112345 (CC BY-SA 4.0).
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