In a small saucepan, heat the water, vinegar, and sugar until the sugar is fully dissolved.
Pack the onions into a jar and pour over the hot liquid. Set aside to cool.
Preheat the oven to 400°F(200°C). Line a baking sheet with parchment paper.
In a bowl, combine the bread, olive oil, and garlic powder. Mix to coat the bread. Spread the bread in an even layer on the baking sheet and bake for 10-15 minutes, or until the croutons are crispy and golden.
Heat a grill or griddle pan over a medium-high heat and oil the grate. Grill the peaches for a few minutes per side, until slightly charred.
In a small bowl or jar, combine the olive oil, vinegar, and mustard. Stir or shake until combined.
On a serving tray, arrange the arugula, tomatoes, and basil leaves. Then add the croutons and peaches.
Drizzle on the dressing and season with salt and pepper.