Place rack in upper third of oven; preheat oven to 450°F.
Toss brussels sprouts, pancetta, garlic, and olive oil on a large baking sheet. Season with kosher salt and freshly ground black pepper and spread in a single even layer.
Roast, stirring once halfway through, until sprouts are brown on edges and tender, about 25 minutes total.
Add ¼ cup water and scrape up brown bits from bottom of the pan. Serve warm.
