Ideal bread to use is Pain de Campagne for the crunchiness after you’ve grilled it. Mouthwatering cheeses to use are blocks of Swiss and Gruyere that you will slice to your desired thickness, but I prefer thin so they melt faster.
Slice however many button mushrooms you want and one regular yellow onion, and then saute them with whipped Cabot butter, or regular unsalted butter, in medium heat. Set aside.
You’re now ready to assemble the cheese sandwich by placing the cheese slices on a bread slice and layer it with the sauteed mushrooms, followed by onions, and more cheese slices. Spread whipped butter or pats of regular butter on one side of the sandwich.
While you fry the buttered side in medium heat, spread whipped butter or pats of regular butter on the top side. Be sure to check after a minute or two, careful not to burn it because it toasts pretty quickly. Turn it over with a spatula with your finger on the bread so the insides don’t splatter out. After you finish frying the other side, place the sandwich on a plate and take out your favorite flavored balsamic vinegar and either dip or spoon a few drops on the sandwich and experience Bliss.