Bring the vinegar and honey to a simmer in a small pot over medium heat until reduced by half. Do not boil, turn to medium low if needed. Set aside to cool.
1 Cup Balsamic Vinegar, 1 teaspoon Honey
Make the Caprese Sandwiches:
Prepare the pesto per recipe instructions.
Butter one side of each slice of bread. Then flip each slice and spread a generous layer of pesto on the other side.
Butter, Homemade Basil Pesto
With the pesto side facing up, layer on arugula, basil, sliced tomatoes, and fresh mozzarella. Season with a sprinkle of salt and pepper, and drizzle with a little balsamic glaze. Top with the second slice of bread, ensuring the pesto side faces inward.
Bread, Fresh Mozzarella, Roma Tomatoes, Arugula, Fresh Basil Leaves
Heat a panini grill or grill pan and grill until the bread turns golden and crisp and the cheese melts. Serve immediately.
Notes
This sandwich really should be eaten right away, but leftovers could be kept in the refrigerator for 1 day.