SAUTE VEGETABLES: Heat oil in a large dutch oven or pot over medium heat. Add onion and saute for about 4-5 minutes, until softened and fragrant. Add garlic, cauliflower, broccoli and carrots and cook for another 5 minutes. Season with salt and black pepper. Add 3.5 cups of chicken broth and simmer for 20 minutes.
SIMMER: Add the chicken broth and bring to a boil. Reduce heat and cover and allow to simmer, for 10-15 minutes.
MAKE CASHEW CREAM: Meanwhile, place drained cashews, reserved 1/4 cup chicken broth, lemon juice, nutritional yeast and mustard into a high speed blender and blend until smooth and creamy.
PUREE SOUP: Use an immersion blender to blend soup or pour into a high speed blender, working in batches, as needed. Blend until desired consistency: creamy smooth or with some chunks, as desired.
SEASON AND SERVE: Combine with cashew cream and season to taste, with additional seasonings, as needed. Garnish with optional toppings.