Position a rack in the center of the oven and heat the oven to 350°F. In a 4-quart pot, bring about 2-1/2 quarts of water to a boil over high heat. Add the onions and cook until the water starts to boil again, about 5 minutes. Drain and rinse under cold water. Using a sharp knife, trim off the root ends, and then pinch the onions to slip them from their skins.
In a 10-inch straight sided sauté pan, cook the pancetta over medium heat until the fat has rendered, 3 to 4 minutes. With a slotted spoon, transfer the pancetta to paper towls to drain. Pour the fat into a heatproof cup and reserve.
Add the butter to the skillet along with the shallots and garlic. Cook over medium heat, stirring, until softened, about 3 minutes. Add the cream and simmer until reduced by half, about 10 minutes. Season to taste with salt and pepper. Transfer the onions to a 2-quart, shallow baking dish. Sprinkle with 1 Tbs. of the chives, and cover with the cream.
In a small bowl, toss the panko with the Parmigiano, pancetta, and 1-1/2 tsp. of the reserved fat (discard the remainder). Season lightly with salt and pepper. Spread the crumb mixture over the onions and bake until the crumbs are toasted and the onions are bubbling, about 30 minutes. Turn the broiler on high, and broil until brown, 3 to 4 minutes. Sprinkle with the remaining 1 Tbs. chives and serve.