In a small bowl, whisk together the beef stock, ketchup, Worcestershire sauce, and cornstarch, and set aside.
In a 12-inch skillet, melt the butter over medium-high heat. Add the sliced onion and saute for 2 minutes. Add the diced bell pepper and saute for another 2 minutes. Remove the sauteed onion and pepper from the pan and set aside.
In the same skillet, add ground beef, salt, and pepper. Brown and crumble the beef until cooked through and no longer pink, about 4 minutes. Add the cooked onion and pepper back into the skillet and mix. Lower the heat to medium.
Stir in the prepared sauce and cook over medium heat, stirring constantly until thickened. Stir in the 1 cup of cheese whiz and remove from the heat.
Serve on toasted hamburger buns or other rolls and top with more warmed cheese whiz if desired.
Notes
Add sliced mushroom if desired, and cook with the green bell pepper.
Use sliced provolone instead of cheese whiz if desired. Stir in 6 oz sliced provolone when adding the sauce.
Store any leftovers in the refrigerator for up to 3 days.