Heat the oil in a saucepan over medium heat. Sauté the onion and ginger for 3-5 minutes until they start to soften.
Add the squash and broth. Cover with a lid and bring to a simmer for about 15-20 minutes until the squash is soft.
Using a blender or immersion blender, purée the soup until smooth. Then return to a simmer.
Dissolve the cornstarch in an equal amount of water. Slowly pour it into the simmering soup until it becomes nice and thick. Taste and season with salt and pepper. Garnish with scallions.