Select the sauté function on your Instant Pot. Add plant butter or oil (or a splash of water for oil-free). Add onion, celery, and carrots, cooking until soft. Stir frequently to avoid any bits sticking to the bottom, which could prevent the pot from pressurizing.
Add garlic and sauté for 2 more minutes, stirring often. Be careful not to let the garlic brown—it can turn bitter.
Cancel the sauté function. Add potatoes, vegetable broth, bay leaves, marjoram, salt, and pepper. Stir to combine.
Secure the lid and set the Instant Pot to Pressure Cook on High for 10 minutes.
While the soup is cooking, drain the soaked cashews. Blend cashews with ½ cup water in a high-speed blender until smooth. Set aside.
Once cooking is complete, use a quick release to release the pressure. Open the lid and carefully discard the bay leaves. Use an immersion blender to partially blend the soup, leaving some chunks of potato for texture.
Slowly stir in the blended cashew mixture. Adjust salt and pepper to taste. If the soup is too thick, add a little more broth. Soup will get thicker as it sits.
Ladle soup into bowls and garnish with sliced green onions or chives. Enjoy warm!