Preheat grill to medium high heat, about 375˚F.

In a small saucepan, stir together balsamic vinegar and brown sugar, bringing the mixture to a boil over high heat. Reduce to a simmer on medium low heat. Cook until the vinegar is reduced and thickened, about 10 minutes. Stir in black pepper. Cover and set aside.

In a small bowl, stir together olive oil and garlic powder until combined.

Brush both sides of the bread slices with garlic olive oil.

Once hot, add bread to the grill, toasting on one side, about 2 to 3 minutes. Remove from heat to a baking sheet, toasted side up.

Add 2 to 3 basil leaves to each slice of bread (you will have some extra leftover, set that aside for later). Top with tomato halves and mozzarella.

Carefully add slices of bread back to the grill, grilling until cheese is melted, about 3 to 5 minutes.

Remove from grill. Top with avocado slices and drizzle with balsamic glaze. Sprinkle with red pepper flakes.

Chop remaining basil leaves and sprinkle on top. Serve immediately.