Melt the butter in a small frying pan over medium heat. Add the mushrooms, garlic and thyme. Cook, stirring, for 2-3 minutes or until the mushrooms are soft and slightly golden. Season with salt and pepper to taste. Set aside to cool slightly. Discard garlic clove.
Spoon ¼ of the mushrooms onto a Pane di casa bread slice. Add the mozzarella on top of the mushrooms. Top with a Pane di casa bread slice. Repeat with the remaining Pane di casa bread slices, mushrooms and mozzarella.
Toast the sandwiches until golden. Serve immediately – buon appetito.
8 Pane di Casa bread slices|40g butter|400g Swiss brown mushrooms, brushed and cut in half (alternatively, use Portobello mushrooms)|1 clove garlic, whole|Leave of 2 sprigs thyme|150g mozzarella, sliced|Sea salt, freshly ground black pepper