Put a pan over medium to low heat and add the sliced onions with a knob of butter.
Slowly fry the onions over low heat, stirring constantly, until they are brown, soft and sweet. This step needs a bit of patience. Only over low heat the onions with caramelize and taste sweet and delicious afterwards.
When the onions are nearly done, add the mushrooms and fry until they are browned.
Slice the gruyere cheese and lay out half of it on two slices of bread.
Add the onions and the mushrooms.
Use the hot pan to fry the bacon until crispy and add to the sandwich.
Finish with the remaining slices of gruyere cheese.
Butter the outsides of the bread slightly and fry on both sides until the cheese is melted and the bread is crispy.