Make the bowls: Preheat the oven to 400 degrees F. Use a paring knife to cut a large circle around the stem of each pumpkin (make a zigzag cut, if desired). Remove the lid and scoop out the seeds and fibers. Sprinkle the inside of each with 1/2 teaspoon each sugar and salt. Place the pumpkins and lids on a baking sheet; roast until tender, 20 to 35 minutes, depending on their size.
Meanwhile, make the soup: Melt the butter in a large saucepan over low heat. Add the onion and 1 teaspoon salt. Strip the thyme leaves into the pot, increase the heat to medium and cook, stirring occasionally, until the onion is soft, about 5 minutes. Add the squash and sugar and cook, stirring, until glazed, 3 to 4 minutes. Add 5 cups water and bring to a boil. Reduce the heat to low and simmer, uncovered, until the squash is tender, 15 to 20 minutes.
Working in batches, transfer the soup to a blender, crack the lid to let steam escape and puree until smooth; return to the saucepan (or puree directly in the pan with an immersion blender). Stir in the heavy cream, if desired. Season with salt and pepper, and top as desired.
Photograph by Steve Giralt
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.