Sauteed mushrooms: add a tablespoon of butter and a tablespoon of olive oil to a medium pan over medium heat. Once the butter is melted and starting to foam up, add all of your sliced mushrooms and a pinch of salt and ground black pepper. 

Cook mushrooms stiring often, for 7 minutes or until the mushrooms have started to soften. Add the Marsala wine, stir, and cook for an additional 3 minutes. 

Remove the mushrooms from the heat to a bowl or sealed container. The mushrooms can be kept in the refrigerator for up to a week. 

Grilled chicken: preheat a grill or grill pan on the stove for 10 minutes. Season your sandwich-sized chicken cutlets on both sides and place them on the grill or grill pan. Cook on the first side for about 5 minutes. At this point, if these are fairly thin cutlets, the chicken should be almost all the way cooked through. 

Flip and cook for an additional 2 to 3 minutes on the second side. If you have a probe thermometer, you should be looking for 165 degrees F (73 C).

Once the chicken is almost done, add a spoonful of mushrooms on top of each cutlet and then layer Swiss cheese slices on top of the mushrooms. Close the lid of the grill or add a pan lid or metal bowl over the chicken if using a grill pan on the stove, and cook for an additional 1 to 2 minutes or until the cheese is melted. Remove the mushroom and cheese-covered chicken to a plate.

While the chicken rests for a couple of minutes, slice your buns and toast them on the grill or grill pan. They should only need a minute or two to receive a light toasting. Turn off the grill or stove and start sandwich assembly. 

Sandwich assembly: spread a bit of mayonnaise on the bottom bun. Top with a chicken cutlet covered with mushrooms and cheese. Optionally add mayo to the top bun, add some crispy fried onions, and sandwich everything together with the top bun.