Prepare the Ingredients
Mash the avocado in a bowl with a pinch of salt and pepper.
Slice the tomato thinly and pat with a paper towel to remove excess moisture.
Assemble the Sandwich
Spread the mashed avocado evenly onto one side of each slice of bread.
Top two slices with mozzarella cheese and tomato slices. Add basil leaves or a drizzle of balsamic glaze if using.
Place the remaining slices of bread on top, avocado side facing the filling.
Toast the Sandwich
Heat a non-stick skillet over medium heat. Add butter or oil and let it melt.
Place the sandwiches in the skillet and press down gently with a spatula. Cook for 3-4 minutes per side, or until the bread is golden and the cheese is melted.
Serve
Slice the sandwiches diagonally and serve hot.