Season steak with salt, pepper, garlic powder and onion powder.
In a large skillet over medium-high heat, melt the butter and cook the steak until slightly browned.
Remove the steak and place on a plate. Set aside.
Add the onions and peppers, toss once to coat in the butter and cook 2-3 minutes without stirring. Then add the garlic, stirring occasionally for about a minute. Then stir in Worcestershire sauce and mustard and cook until vegetables have softened.
Top with cheese, reduce heat to low, and continue cooking until cheese has melted. (You can also set skillet under broiler if you wish to brown the cheese)