Turn the Instant Pot to sauté. Add the butter and let melt, then add the onion. Cook until the onion softens and turns translucent, about 5 minutes. Add the garlic and cook 30 additional seconds, just until fragrant.
Add the water to the Instant Pot. Let come to a simmer (this will take several minutes), then whisk in the bouillon. Continue to stir until it is smoothly dissolved.
Add the potatoes, cauliflower, mustard, thyme, and pepper. Cover and seal the Instant Pot. Cook on manual (high) pressure for 10 minutes. When the cooking time is up, immediately vent to release any remaining pressure.
Carefully open the lid. With an immersion blender, purée the soup until smooth, or puree the soup in batches in a regular blender (be careful not to fill the blender more than halfway. Hot soup will splatter). Return any soup you pureed in the blender to the pot. Let the soup cool a few minutes, then stir in the Greek yogurt, cheddar, and Parmesan.
Taste and adjust the seasoning as you like. Serve hot, topped with chives, additional cheese, and a dollop of Greek yogurt as desired.
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Instant Pot Potato Soup https://www.wellplated.com/instant-pot-potato-soup/