MIX together ½ cup crushed tomatoes, garlic, red pepper flakes and oregano in a medium bowl. Set aside.
BUTTER the outside of each piece of bread.
SLICE the fresh mozzarella into ¼" slices.
SPREAD tomato mixture on the inside of one piece of bread.
LAYER mozzarella on top of tomato mixture. Then add basil and pepperoni (if using).
TOP with final slice of bread.
HEAT pan on medium-low heat.
PLACE your sandwich into the pan.
FLIP the sandwich when the bread starts to turn golden brown and cheese has begun to melt.
REMOVE from the pan once both sides of bread are toasted and the cheese is fully melted.