Butter the brioche bread slices thoroughly.
Mix the grated Parmigiano-Reggiano and shaved Gruyere cheese.
Dip the buttered side of the bread into the cheese mixture to coat.
Heat a non-stick pan over low heat.
Place the coated bread slices in the pan and cook until lightly golden brown.
Add the mozzarella cheese slices and place under the broiler for 8-10 seconds to melt the cheese.
Fold the sandwich and let it cool for 90 seconds before serving.
