Preheat BBQ to 350℉, using indirect heat method.
Place tart shells on a baking sheet.
In a large bowl, whisk ½ cup brown sugar, cornstarch, cinnamon, and nutmeg together.
Stir bourbon, lemon juice, and vanilla extract.
Add diced peaches and stir to coat.
Fill tart shells with peaches.
Place on BBQ away from heat.
Bake for 15 minutes or until peaches are tender.
Remove from BBQ and allow to rest for 5 minutes.
Sprinkle the 3 Tbsp of brown sugar evenly over hot tarts.
Serve warm.