Makes 1 1/2 quarts; serves 4 to 6
Melt butter in a large saucepan over medium-high heat and add onions, parsnips, and celery. Cook until vegetables are lightly caramelized around the edges, 4 to 6 minutes.
Add garlic and thyme and cook, stirring, until garlic is fragrant, 1 to 2 minutes.
Add turnips, stocks, salt, and pepper and bring to a boil. Reduce heat just to a simmer and cook until turnips are very tender, 20 to 25 minutes.
Remove thyme and add heavy cream, if using.
Using an immersion blender (or in batches in a blender), puree soup until very smooth.
Rewarm if necessary, then serve in small bowls, garnished with a dollop of sour cream, a pinch of chives, and parsnip chips.