Heat the water, vinegar, sugar, and salt to a boil in a saucepot over high heat; remove from heat and cool 8 minutes or to 160°F. Add onion and let steep for at least 2 or up to 6 hours.
Drain onion, reserving 2 tablespoons of the pickling liquid in a bowl; whisk in oil. Reserve vinaigrette for serving.
For the crostini:
Brush both sides of baguette slices with oil. Grill baguette slices over medium-high heat 3 minutes or until grill marks appear, turning once; sprinkle with salt and pepper.
For the grilled fresh peaches:
Evenly brush peach wedges with oil; grill cut side down 5 minutes or until
grill marks appear, turning once.
Top 3 crostini with 2 tablespoons cheese and heat on a sheet tray at 350°F for 2 minutes or until cheese melts.
Toss ½ cup arugula with 1 tablespoon vinaigrette in a bowl; evenly divide over cheese-topped crostini. Evenly top with pickled red onion, 6 peach wedges and 1 tablespoon almonds; drizzle with balsamic glaze.