In a large bowl, add all the hamburger patty ingredients. Mix, but don’t overwork. Over mixing the meat makes a tough hamburger.
Separate into 8 sections, roll each one into a ball and flatten into a patty. Set aside.
Preheat a medium skillet to medium-low heat, add oil, then add onions. After about 3 minutes, add mushrooms. Cook onions and mushrooms until the onions are nice a golden brown and the mushrooms are cooked through, about 10 minutes. Season with salt and pepper.
Heat an indoor grill, skillet pan or barbecue grill to medium-high. Add hamburger patties. Turn to medium. Cook until browned, about 3-5 minutes. Flip. When almost completely browned, add cheese. Cook until cheese is mostly melted, and to your desired doneness.
I like to either toast the buns on the grill or warm them in the oven or microwave.
Put mayo on the bottom bun and top it with lettuce, tomato, the hamburger, onions and mushrooms, and spread barbecue sauce on the top bun.