Dry brining burger patties with onions I tend to mix in some onions when making burger onions, before putting on a rack and letting it sit in the fridge for an hour before cooking. With that said, if I dry brine at the same time, will the salt interact with the onion already in the meat during that hour?
Salt mixed into hamburger results in patties that have the consistency of sausage instead of a good burger.

Brining is totally unnecessary for ground meat. It is a mechanism for conveying flavor into meat, helping reduce moisture loss, and increasing the illusion of tenderness. None of these are necessary with ground meat because it is ground! It will already be tender and flavorings can be mixed right in.

Don't mix salt into the beef. Apply it only on the surface when it is cooked.