Step 1

Position a rack in the middle of the oven and preheat to 400 degrees. Line a large sheet pan with foil or parchment paper.

Step 2

Pour the oil on the prepared pan. With a sharp knife, cut off the very top of the head of garlic, exposing each of its cloves. Use the head, cut side down, to spread the oil around the pan, then wrap the garlic in a small piece of foil and set it in one corner of the pan.

Step 3

Spread the onions evenly over the rest of the pan. Sprinkle evenly with the salt, sugar and onion powder, and use your hands or tongs to toss the onions so they are evenly seasoned. Roast for 20 minutes. Using tongs, toss the onions, moving any browned onion slices toward the center and undercooked onions to the sides. Return to the oven and continue roasting for another 20 to 25 minutes, or until all of the onions are at least lightly browned and very soft.

Step 4

Transfer the onions to a blender. Unwrap the head of garlic and squeeze the roasted cloves into the blender. Add 2 cups of the broth. Remove the center ring from the lid, hold a kitchen towel over the lid and blend until smooth, 2 to 3 minutes. For a thinner soup, add more broth. Taste, and season with additional salt and/or pepper as needed.

Step 5

Transfer the soup to a medium saucepan, set over medium heat and bring to a gentle simmer.

Step 6

To serve, divide the soup between two bowls, and top each with a dollop of sour cream, some potato chips and chives.