Butter one side of each bread slice.
Place four slices butter-side down in skillet.
Spread pizza sauce on unbuttered side.
Add mozzarella cheese evenly on sauce.
Layer pepperoni and mushrooms on cheese.
Sprinkle Italian seasoning on toppings.
Top with remaining bread slices, butter-side up.
Cook on medium heat for 3-4 minutes.
Flip sandwiches carefully using spatula.
Cook another 3-4 minutes until golden brown.
Remove from heat and cool slightly.
Cut diagonally and serve immediately.