Step 1Slice the fillet crosswise into thin strips about one-eighth inch thick. Set aside.
Step 2Spray a medium pan with nonstick spray and heat over medium-high heat. Add the mushrooms and onions and cook, stirring occasionally, until the onions are slightly browned, about 5 minutes. Remove from heat to a small bowl and set aside.
Step 3Re-spritz the pan with nonstick spray. Cook the fillet strips over medium-high heat until browned on each side, 1 to 1 1/2 minutes on each side.
Step 4Break the cheese slice into small strips, place on top of the meat -- still in the pan -- and continue to cook until the cheese has melted slightly, less than 1 minute.
Step 5Remove from heat and mix the cheesy beef strips in with the mushrooms and onions. Serve in the lettuce "cups."