Soak or boil the cashews until they are soft and puffy (see note below), then drain and rinse and put in the blender.
Make the cream sauce. Add the ½ cup of vegan sour cream, ½ cup water, and ½ teaspoon salt to the blender and blend on a high setting until the cashews break down and become smooth and creamy. Approximately 2 minutes. Scrape the sides and blend once more. There shouldn't be any solid pieces of the nuts in the cream sauce. Set aside
Over medium heat, heat a large stock pot and add the olive oil. When the oil is hot, add the carrots and celery and cook until the celery begins to soften. Approximately 5 minutes. Stir frequently.
Add the minced garlic and stir. Cook for 2 minutes, stirring frequently.
Add the potato, broccoli, and the ¼ cup of nutritional yeast to the pot and pour in the 64 ounces (two 32 ounce containers) of vegetable broth. Stir to combine and bring to a boil.
Once the soup is boiling, reduce the heat to a simmer and cook until the vegetables are fork tender. Approximately 10 to 15 minutes.
Whisk in the cashew cream sauce until fully combined.
Blend the vegetables with an immersion blender until the vegetables are blended into small pieces, or to your desired texture.
Top with any of the suggested toppings.