Turn on the Saute function on your slow cooker. Heat vegan butter and olive oil. Add leeks, onion, carrot, and mushrooms. Cook and stir until leeks and onions are translucent and softened, carrots have softened, and minced mushrooms have liquefied, about 7 minutes. Add potatoes and vegetable broth; mix well. Mix 1 tablespoon of the oat milk with potato starch to create a slurry and stir it into the soup.
Heat remaining oat milk in a microwave-safe bowl in the microwave until hot, about 30 seconds. Pour it into the soup and season with salt and pepper. Turn off Saute function.
Cook in the slow cooker on Low for 6 hours or on High for 3 1/2 hours. Stir soup before serving.
If your slow cooker does not have a saute function, saute the ingredients in a skillet and then transfer to the slow cooker to proceed. Do not skip the saute method. It is vital in creating a robust and rich flavor.
If you don’t want to use oat milk, use any unsweetened, neutral-flavored, full-bodied vegan milk. Rice milk is not a good substitute. Flax milk would be a good substitute.
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