In this lightened version of the Italian dish chicken agrodolce ("sweet and sour"), a grouping of several small alliums -- cipollini, pearl onions, and shallots -- are coated with a glaze made from red wine, vinegar, and honey.
Bring 2 inches of water to a simmer in a large stockpot. Set a steamer insert in pot, and add cipollini, pearl onions, and shallots. Cover, and steam until tender, 10 to 12 minutes. Transfer to a plate, and let cool slightly, about 10 minutes. Peel cipollini and pearl onions, leaving tops and root ends intact; set aside.
Stir wine, vinegars, honey, and 2 teaspoons salt in a small bowl; set aside.
Heat 1 tablespoon oil in a large skillet over medium heat. Add cipollini, pearl onions, shallots, bay leaf, and rosemary sprig. Cook, stirring often, until cipollini, onions, and shallots are deep golden brown in spots, about 5 minutes.
Add vinegar mixture to skillet. Reduce heat to medium-low, and simmer, stirring occasionally, until liquid is syrupy, about 30 minutes. Discard rosemary and bay leaf. Season with pepper.
Meanwhile, preheat broiler, with rack 6 inches below heat source. Butterfly chicken: Using a chef's knife and starting on one long side, slice chicken open horizontally, stopping about 1/2 inch before reaching opposite side. Open each piece of chicken like a book, and place it between two pieces of plastic wrap. Pound chicken to 1/4 inch thick.
Coat a rimmed baking sheet with cooking spray. Brush chicken on both sides with remaining tablespoon oil, and season with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Transfer to the prepared sheet; broil chicken, flipping once, until golden brown and cooked through, about 3 1/2 minutes per side.
Transfer chicken to a serving platter, and add cipollini, pearl onions, and shallots. Garnish with rosemary sprigs.