Spread one side of each bread slice with butter; arrange mozzarella and other toppings (if using) on unbuttered sides of bread slices. Sandwich together.
Heat large nonstick pan over medium heat. Cook sandwiches, flipping once, until golden brown and cheese is melted, about 6-8 minutes. Remove sandwiches; sprinkle half of Gruyère in two lines in pan. Press sandwiches into cheese. Cook until cheese is golden. Remove sandwiches; repeat with remaining cheese on other side. Serve with DIY Ketchup.
Tip: Add a few splashes of whiskey or port while caramelizing onions for a big boost of flavour!