Warm the olive oil (or butter) in a Dutch oven over medium heat. Add the onion and cook, stirring, until softened, about 5 minutes. Add the garlic and cook 30 seconds, just until fragrant.
Stir in the thyme and cumin. Add the pumpkin purée and cook for 1 minute to toast the spices slightly.
Pour in the vegetable broth and coconut milk. Bring to a gentle simmer, then reduce heat to low and simmer 15–20 minutes, stirring occasionally.
Blend until smooth using an immersion blender (or carefully transfer to a blender in batches). If the soup is too thick, add a splash of broth to reach your desired consistency.
Season with salt and pepper to taste. Keep warm over low heat while you make the topping.
In a small bowl, mix the heavy cream (or sour cream), chopped cilantro, lime juice, and a pinch of salt.
Ladle soup into bowls and swirl in the cilantro cream. Garnish with extra cilantro and pumpkin seeds if you like. Serve hot.