Preheat oven to 300°F. Line a baking sheet with parchment paper or a silicone baking mat.
Combine sugar and fennel seeds in a bowl. Place pancetta in a single layer on prepared baking sheet; rub spice mixture evenly on each side. Bake at 300°F for 25 minutes or until crisp.
Combine vinegar, honey, and juice in a small saucepan; bring to a boil over medium-high. Cook, stirring often, 6 to 8 minutes or until mixture thickens and reduces by half.
Cut each peach in half horizontally; discard pits. Cut peach halves in half horizontally. (You should have 4 round, flat slices from each peach.)
To assemble, place 6 peach slices on a serving platter. Layer each with 1 mozzarella slice, 1 additional peach slice, and 1 pancetta slice. Drizzle vinegar mixture evenly over stacks. Sprinkle with basil.