Heat a large frying pan over medium-high heat. Add the butter to the pan then add the mushrooms.
Allow the mushrooms to fry until golden brown then add the garlic, chilli and oregano. Allow to cook for another minute then season with salt, pepper and lemon juice. Remove from the heat and set aside.
To assemble the sandwiches, mix the mozzarella with the cheddar. Divide the cheese between four slices of bread. Top with mushrooms and sandwich with the remaining slices of bread. Generously butter both sides of the sandwich .
Preheat a non-stick pan over medium heat. Add the sandwiches and cook until the cheese has melted and the sandwiches are golden brown and crisp on both sides.