Prepare burger seasoning, and use for the recipe. In a small bowl combine the kosher salt, ground black pepper, and garlic salt.
Saute onions with butter on the grill. Sprinkle burger seasoning on the onions. Cook the onions until they are tender. Do not brown the onions.
Butter each of the buns and cook the buns on the grill. Cook the buns on the grill until they are lightly brown.
Form 8 balls of the burger meat. Heat the grill to about 350 degrees. Place the balls on the grill. Smash burger meat onto the grill. Season with burger seasoning.
Cook for 2 to 3 minutes flip and continue to cook on the other side. Place 1 slice of Swiss cheese on each burger while they are on the grill.
Build the burger by building up from the bottom by adding the two burgers with cheese on each bun. Then top with grilled onions, and spread some Boursin cheese on the top bun. Top the burger with the top bun.