Step 1

Heat butter in a large saucepan or Dutch oven over medium-high heat until melted. Continue cooking, swirling pan constantly, until butter solids are lightly browned and the butter smells nutty, about one minute longer. Immediately add onion, celery, carrot, and whole thyme sprigs (if using), reduce heat to medium, season with salt and pepper, and cook, stirring occasionally, until vegetables are softened but not browned, about 5 minutes. Add stock, squash, and bay leaves. Increase heat to high and bring to a boil. Reduce to a bare simmer and cook until squash is completely tender, about 20 minutes.

Step 2

Using tongs, discard bay leaves and thyme stems. Add heavy cream. Using an immersion blender or working in batches in a countertop blender, blend soup until completely smooth. Season to taste with salt, pepper, and lemon juice. Add sugar if desired (some squashes are naturally sweeter than others). Serve.