Slice the onions into half rings and slice the mushrooms into thin pieces. On high heat, fry the mushrooms and onions until golden brown and season them with salt and pepper. Set the veggies aside.
In a large bowl, mix the ground beef and seasoning together with your hands. Form burger patties about 3/4-inch thick, similar in size to burger buns.
Pop a couple of patties into the same skillet that you cooked the onions and mushrooms and turn the heat to medium. Cook pressing down, one each side for 5 minutes or until internal temperature reaches 165F. Add cheese towards the end of cooking time.
Start with bun, followed by a burger patty and a handful of the onion and mushroom mixture.
Cover the top bun with mayo and ketchup and pop it on top. Repeat until all your burgers are dressed.
Let the burgers rest for a min so they can set a little. Enjoy!