Dice the onion and green pepper. Slice the mushrooms and mince the garlic.
Cook the ground beef in a large skillet over medium high heat, letting it sear on both sides before breaking it up.
Turn the heat to medium and add the onion. Cook for 3-4 minutes or until the onion starts to get tender.
Add the green pepper and mushrooms, cook and stir for 2-3 more minutes.
Stir in the garlic, worcestershire, salt, pepper, and bouillon.
Whisk together the water and cornstarch, then pour into the skillet.
Cook for 3-4 minutes uncovered, or until slightly thickened.
Arrange the cheese slices over the top of the meat mixture. Turn off the heat and cover the skillet with a lid. Let sit until the cheese melts.
While the cheese is melting, spread the buns with butter. Either toast them in a hot skillet, or place them on a cookie sheet butter side up and broil them until lightly browned.
Divide the meat mixture between the six buns. Serve immediately.
Notes
Buttering and toasting the buns is optional, but so worth it!