Spread butter on one side of each piece of sourdough bread.
Place a medium skillet or griddle over medium heat. Once the pan has pre-heated for 4 or 5 minutes add the slices of bread, butter side down. Toast for 2 to 4 minutes or until the outside surface of the bread is golden brown and toasty. You can toast the other side of the bread if you want, but it's not required.
Raise the temperature under your pan/griddle to medium-high
Once the bread is toasted to your liking, form the third pound of ground beef into a thin patty (about a 3/4-inch thick) that is a little bit wider than your toast (or as close as you can get). Salt and pepper the ground beef patty and place the seasoned side down in the hot pan. Season the second side and allow the patty to cook, untouched, for 3 to 4 minutes.
Flip the patty and add one or two slices of Swiss cheese on top of the patty. Cover the pan to help the cheese melt and cook the second side for at least 2 minutes. If you're worried about undercooking the burger, cook the second side for an additional 2 to 3 minutes.
Remove the patty to a plate while you assemble the rest of your burger.
Add mayonnaise and the second slice of cheese to the inside of the bottom slice of bread. Top the cheese with your tomato slices. Season the tomato with a tiny pinch of salt and ground black pepper.
Top the tomato slices with the burger patty and cheese. Add two slices of bacon and top with the second slice of sourdough that you have also spread mayonnaise on.
Serve and enjoy.