Heat the oven to 400°F (200°C).
Tear the bread into bite-size pieces and toss with 1 tablespoon/15 ml olive oil. Spread evenly on a baking sheet and bake for about 10 minutes, until crisp and golden. Let cool completely, then crumble into coarse breadcrumbs by hand.
In a large bowl, combine the cucumber peels, scallions, red onion, cilantro, basil, mint, pine nuts, sumac, salt, olive oil, and lemon juice. Toss well to form a chunky, herb-packed salsa.
In a medium skillet, heat the remaining 2 tablespoons/30 ml olive oil over medium heat. Add the challah crumbs and cook, stirring often, for 5 to 7 minutes, or until deeply golden and crisp. Transfer to a paper towel-lined plate to cool.
To assemble, arrange the watermelon slices on a large serving platter. Spoon the herb salsa generously over the top. Add a few slices of green chili and scatter with crumbled feta. Drizzle lightly with olive oil and finish with a handful of the crisp challah crumbs. Serve immediately.