Pre-heat the oven to 375. Lightly mist 8 cups in a muffin tin with cooking spray and set aside.
Mist a large skillet with cooking spray and bring to medium heat. Add chopped onions and sauté until cooked. Add ground beef and Hamburger seasoning and cook until meat is browned. Add mushrooms and continue to cook, stirring occasionally, for a few minutes until mushrooms are browned as well.
In a microwavable bowl, combine meat/mushroom mixture, cheese wedges, ketchup and mustard and stir. Microwave for one minute on high. Stir to combine until meat mixture is fully coated.
Push a wonton wrapper into the bottom of each of the eight sprayed cups in the muffin tin. Using about half of the meat mixture, spoon evenly into the wonton wrappers. Follow by sprinkling some of the chopped tomatoes into each cup (using about half the total tomatoes). Sprinkle about half the cheddar cheese evenly over the top of each cup. Press another wonton wrapper on top and repeat the layering steps with the remaining ingredients.
Bake for 18-20 minutes until golden brown. Let cool 5 minutes before removing from muffin tin.