In a large skillet, on med high heat, brown the ground beef, breaking pieces apart while it is cooking.
While browning the ground beef start chopping up the green pepper and mushrooms into pieces and finely chop yellow onion, set aside.
Remove beef from skillet, set beef aside. Drain all but 1/4 cup oil from skillet.
Place green pepper and onion into skillet and cook for about 2-3 minutes. Add in mushroom pieces and cook until mushrooms reduce down.
Place cooked ground beef back into pan with veg mixer. Mix in can of sloppy joe sauce. Remove from heat.
Preheat oven to 350 and lightly spray bottom of a 9x12 casserole pan with non-stick spray.
Create layers as follows in prepared casserole dish: 1/3 of the beef mixer, then a single layer of tostito chips, then a layer of shredded cheese on top of chips. Repeat layers 2 more times ending with cheese on top. You will most likely have some chips left over, if so set them aside for later.
Place a lightly sprayed piece of foil with sprayed side down on top of casserole dish. Cook 20-25 mins in the preheated oven.
Remove from oven and let set for a few minutes before cutting into serving squares. Place a few exta chips on each plate when serving.