Open your Griddler so that the griddle plates are open flat. You want to use the smooth griddles.
Turn the Griddler Selector knob (the center one) to 'griddle'. Set the temperature know to 425 F, the maximum.
Once the green light is lit, indicating that the Griddler is at 425 F, you can being cooking.
Place the bell peppers and onion in a small bowl. Drizzle with the oil and season with salt and pepper. Stir and transfer to one side of one of the griddle plates. See my picture for how I arrange things on the Griddler.
On the other plate place the bread halves. Let them toast as long as desired as the rest of the ingredients cook.
Let the onion and peppers cook for 5 minutes, stirring a few times.
Add the mushrooms to the onion and peppers. Stir and cook another 5 minutes. The vegetables should be just starting to turn tender.
Place the Steak-umm on the griddle plate alongside the vegetables. Season as desired.
Continue cooking the Steak-umm until it starts to brown along the edges. Flip and continue cooking until done.
Assemble the sandwich by placing the Steak-umm on the bottom bread half. Add the cooked vegetables. Drizzle on the cheese, add the bread top.
Serve with plenty of napkins. You want that cheese to be nice and runny!