Add onion to the pot and sauté, stirring occasionally, until it becomes soft and translucent, about 3 minutes. Stir in 1 cup broth and scrape the bottom of the pot to deglaze.
Add potatoes, cauliflower, remaining chicken stock, milk, salt, pepper, and garlic powder. Attach lid and securely seal, making sure valve is in “sealing” position.
Use an immersion blender to blend the soup to the desired consistency, or transfer it in batches to a blender to puree. Taste and add additional salt, pepper, or garlic powder, if desired.