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 Kraft Burger Seasoning together with your hands. Form burger patties about 3/4-inch thick.
Pop a couple of burgers into the same skillet that you cooked the onions and mushrooms and turn the heat to low. Cook one side for 5 minutes and the other side for 4 minutes.
Place a slice of cheese on the bottom 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 2-3 minutes so they can set a little. Enjoy!