Bring 2 quarts water to boil in large saucepan. Fill large bowl halfway with ice and water. Add 1 pound pearl onions, root ends trimmed, to boiling water and cook for 30 seconds. Drain onions, then transfer to prepared ice bath; drain again. Squeeze tips of onions to pop them out of outer skin.

Combine onions, 8 ounces small shallots, peeled; 1¼ cups water; and ½ teaspoon table salt in 12-inch skillet and bring to boil over high heat. Cover; reduce heat to medium; and cook, stirring occasionally, until paring knife meets little to no resistance when slipped into center of alliums and most liquid has evaporated, 18 to 20 minutes.

Uncover and add ½ cup white wine vinegar, 2 tablespoons packed brown sugar, 2 tablespoons raisins, and ¼ teaspoon pepper. Increase heat to high and continue to cook until liquid has thickened to consistency of heavy cream, 4 to 6 minutes.

Off heat, add 3 tablespoons unsalted butter, cut into 3 pieces, and greens of 4 scallions, cut into 1-inch pieces, and shake and swirl pan until butter is melted, about 1 minute. Transfer to small serving dish and serve.To Use Frozen Pearl Onions: Skip step 1. In step 2, decrease water to 1 cup and add ⅛ teaspoon baking soda along with salt and bring to boil over high heat. Add 3 cups frozen pearl onions along with shallots and return to boil. Boil for 30 seconds. Stir in 1 tablespoon white wine vinegar, cover, and cook for 12 to 15 minutes. In step 3, add remaining 7 tablespoons vinegar along with sugar, raisins, and pepper