1. Put the cubed potatoes in a large pot and add water to where the water level sits about 1/2" above the potatoes. Set them on high and turn your attention to the onions. Cook the chopped onions in a medium pan with a tablespoon of olive oil until they are limp, but not burned.
2. At this point, the potatoes should be boiling heavily. Add the cooked onions to the pot. Then add the celery salt and the curry. Let the mixture cook another 8 to 12 minutes, depending on how you like your soup. The longer you cook it, the smaller your potato cubes become and the thicker your soup gets. Drop the heat down to medium.
3. Put cornstarch and nutritional yeast (if using) in a bowl, add 1/2 cup water, and whisk the mixture until it is smooth. Add this to the soup pot and mix well. It's going to get really thick really quick, so you need to add the milk fast. Mix well.
4. Let it sit on medium for around five minutes, but you don't want the milk to scald or separate, which doesn't really mess with the taste, but it makes it look ugly. Drop the heat to simmer and let it chill out for like 10 minutes.
5. After it's done cooking, ladle out the soup into a bowl and drop 1 teaspoon of vegan butter in it. Shake on some black pepper and paprika, if using. As the vegan butter melts, it forms a little yellow pool and looks real pretty.