Preheat your Yoder Smokers YS640 setup in direct grill mode (or other cooker) to 350°. Place a griddle on the grates, or dropped in place of grates if an option.
Using a skillet over medium heat, melt 2 tablespoons of butter and sauté your diced onions. As the onions cook, season the ground beef liberally with Cattleman's Grill Steakhouse Seasoning. Roll the beef into 1/3 pound balls.
Remove the onions from heat and set them aside. On the grill, put 2 tablespoons of butter on the griddle and allow it to melt. Place the rolled beef onto the grill and let them cook for about 30 seconds before flipping them over. Next, take a cast iron grill press and smash the burger as thin as possible. Cook for another few minutes until they reach the desired temperature (150° to 155° for medium to medium well, use the Maverick PT-75 instant read thermometer for fast and accurate temperature readings).
Just before the burgers are done, place equal parts sautéed onion on each patty and top with a slice of mild cheddar cheese. Once the cheese has melted, you are ready to serve!