Preheat your oven to 400 degrees.

To make the balsamic reduction, add the balsamic vinegar and the honey to a small saucepan and bring to a simmer. Simmer over low heat until thickened and reduced. (Don't let it get too thick, just thick enough to drizzle it on the sandwiches).

Spread the pesto sauce inside the sandwiches. (About 2 Tbsp per side)

Top the sandwiches with fresh mozzarella and tomatoes.

Drizzle some balsamic reduction sauce over the mozzarella slices.

Place the sandwiches halves in a baking sheet and bring them to the hot oven, baking until the cheese melts (around 10 minutes).

Close the sandwiches and serve them warm.