Melt a little bit of lard in a pan over a medium heat and sauté the onion for a minute or two until it start to become translucent.
Add the mushrooms and turn up the heat. Sauté until the onions and mushrooms are nicely caramelized. Approximately 5 minutes.
Add the green pepper and zucchini and sauté for a couple of minutes until it reaches your desired level of tenderness. Season with salt and pepper.
Finally, tear the sliced of roast beef into smaller bit and add to it, stirring until everything is nicely hot throughout.
Sprinkle the cheese on top and serve with a little squirt of mayo or ketchup.