Thaw coho salmon meat.
Add the salmon meat to a bowl and season with your favorite all purpose seasoning. Whisk one egg and mix it into salmon until combined. Form into 8, 2 ounce balls.
Before cooking the patties, caramelize the onion by chopping it and sautéing in butter until golden and soft.
Heat butter in a cast-iron skillet or griddle over medium-high heat.
Place a ball of salmon meat on the hot griddle. Place a piece of parchment paper on top. Use a sturdy spatula (or a burger press, if you have one) to firmly smash the ball into a thin patty, about 1/4 inch thick.
Cook the patties for 1-2 minutes until the edges are crispy and browned. Flip the patties and immediately place a slice of cheese on one and caramelized onions on the other. Cook for another 1-2 minutes until the cheese is melted and the patty is cooked to your desired doneness.
Toast the burger buns on the griddle for a few seconds. Place two cooked patties on the bottom bun, add your desired toppings, and then top with the other half of the bun.
Serve the smash burgers hot with your favorite sides, like fries or a salad.
Enjoy!