Chop onion into thin strips. If you want mushrooms, chop into small pieces. Heat butter/oil over high heat. Add onions and evenly coat with the oil from the pan. Stir often. You may want to add water a few times as the onions are cooking to keep them soft. After a few minutes, add the mushrooms to the pan. Cook until the onions become very tender and start to get stringy. Scoop onions and mushrooms into a bowl and set aside.
Cut Impossible into thin strips with a sharp knife. This works best while the”meat” is still partly frozen. If your Impossible is thawed, you can get the correct texture by pressing it into a thin patty. Heat a little more oil in your pan and add your Impossible strips. Once edges start to appear gray/brown, flip the strips and cook on the other side.
Add the onions and mushrooms back into the pan. Cut the Velveeta into small pieces and place in pan. Spread cheese over the top and then fold your mixture so that your ingredients are evenly distributed. Remove from heat.
Serve on a roll or eat it with Wheat Thins (this is my favorite way to eat it!)