The key to the burger is keeping it simple and not over-working the beef. Start by breaking apart the meat using your fingers. Don't mash the meat, but rather work your fingers into it churning at the wrist to break it into small pieces.
Add the 1/8 teaspoon of coarse ground black pepper to the meat. If the meat was leaner than 80/20, add a tablespoon of vegetable oil. Optionally, if the meat isn't very fresh, but still edible (maybe it's a little gray rather than nice and read, but smells fine) you can try adding a tablespoon of Worcestershire sauce to up the umami flavor. Mix the meat lightly to incorporate the ingredients, but don't overdo it. Please note that I've deliberately left the salt out until later!
Split the meat into two equal portions and lightly form them into patties that are roughly inches across. Again, the key is not to pack the meat too tightly. You should be able to pick up the patties without them breaking apart, but they shouldn't be too dense. The next big tip is to make the patties about 1 inch larger than the hamburger buns you are using. The reason is that the meat will shrink while cooking. Making the patty larger means it will fit the bun properly after cooking. Lastly, when making the patties, form a shallow, circular indent in the center of the patties. The small, shallow indent helps keep the patty's shape after cooking because meat tends to contact into the center while cooking.
Only when you are ready to grill, sprinkle roughly a half teaspoon (or as much as needed) of Kosher salt on the outside of the burger patties.
Get the grill hot enough so that you can only hold your hand over the grill grates for a few seconds. With a propane grill, just set the dials to high and wait for the grill to get up to 550 or 600 degrees F with the lid closed. Place the patties using a spatula on the grill directly over the flame. With a charcoal grill, it may be harder to gauge the temperature, so don't cover the grill if it seems very hot. With a propane grill go ahead and close the lid if you have some kind of temperature gauge on it. Let the burgers grill for 4 minutes (with a closed lid for a propane grill), but never letting the grill temp get above 550 or 600 degrees F with the lid closed (i.e. open the lid to cool down or dial back the heat if needed). After four minutes, flip the patties and let them grill for up to 3 minutes for medium or 4 more minutes depending on how well done you like the patties.
After flipping and when there is roughly 1.5 minutes left, add the Swiss (or whatever cheese you like) to the patties and close the lid again.
Before the burgers are done and if you like a toasted bun, place the burger buns face down on the grill over indirect or medium heat for a minute or two until they are toasted to your preference. If you want to take them to the next level, melt a tablespoon of butter in a small bowl in microwave and brush a light layer of butter on each interior side of the bun before grilling. Set the buns aside when toasted so that they are ready for the burgers.
After the cheese is nicely melted and the burgers have a nice color and crust on them, remove them from the grill and place directly onto the burger buns. Alternatively, you can serve the burger patty over a salad and use the aioli, mushrooms, and onions as toppings.
When it's time to assemble the burgers, you can either place the mushrooms and caramelized onions on the bottom bun with the burger on top or on top of the burger and cheese. I suggest spreading the aioli on the burger bun half that is not touching the onions and mushrooms. This lets both sides of the burger have a unique flavor.
After assembly, the burgers are ready to eat immediately. Enjoy one of the most tender and juicy restaurant or pub style burgers you can make at home.