Slice the onions into half rings and the mushrooms into thin pieces. On high heat, sauté the veggies until they are golden brown. Season to taste with salt and pepper, then set aside.
In a large bowl, mix the ground beef and seasoning together with your hands. Form burger patties about 3/4-inch thick, about the size of the burger buns.
Place a couple of patties into the same skillet used for the veggies and decrease the heat to medium. Cook while pressing down on the patty, about five minutes per side or until the internal temperature reaches 165°F. If desired, add the cheese close to when the meat is all done.
Assemble the burger by starting with the bun, then layering on a cheesy burger patty, a handful of the onion and mushroom mixture, and topping with the top bun dressed in your choice of condiment. Repeat until all the burgers are dressed.
Let the burgers rest for a few minutes, then enjoy!