For the soup: Heat the olive oil in a large pot over medium-high heat. Add the onion and cook until beginning to soften, about 5 minutes. Add the squash and cook, stirring occasionally, until beginning to brown around the edges, 8 to 10 minutes. Add the garlic and cook for another minute. Pour in the chicken stock, add the thyme and sage and bring to a simmer. Simmer until the squash is very soft, 10 to 15 minutes. Stir in the heavy cream. Puree using an immersion blender. Season with salt and pepper. Serve immediately, or let cool completely, pour into freezer bags, label and freeze.
For the topping: When read to serve, mix together the cream and sour cream in a small bowl. Drizzle designs over the soup with the mixture.