Put the oil in a large pot over medium heat. When it’s hot, add the onion and cook, stirring occasionally, until it begins to soften, 2 to 3 minutes. Add the pumpkin and sage, sprinkle with salt and pepper, and cook until fragrant, another minute or so.
Add the stock and bring to a boil, then lower the heat so the soup bubbles gently but steadily. Cover and cook, stirring occasionally, until the pumpkin starts to fall apart, 20 to 30 minutes.
Use an immersion blender to purée the soup in the pot. Or let the soup cool a little, carefully purée it in a blender (working in batches if necessary), and return it to the pot. (You can prepare the soup in advance up to this point. Cool, cover, and refrigerate for up to 2 days. Add the cream and serve cold or proceed with the recipe.)
Add the cream and gently heat the soup without letting it boil. Taste and adjust the seasoning. Garnish with an extra grinding of pepper if you’d like, and serve.