1/2 lemon, sliced very thin with a sharp knife
1/2 teaspoon sugar
8 thin slices white bread
8 ounces fresh mozzarella, thinly sliced (see Cook's Note)
3 tablespoons unsalted butter
1/2 cup freshly grated Parmesan cheese
If using no-salt or low-salt mozzarella cheese, the filling may need to be seasoned with kosher salt.