Using powdered onion and/or garlic? Please do not cook these as described in steps 2 and 3. Add them when you add the vegetable broth. If you don't have fresh onions, you can use 1 tablespoon of onion powder and if you don't have fresh garlic, you can use 1 ½ teaspoons garlic powder. If you have jarred minced garlic, use 4 teaspoons.
Cook the onion: Start by sautéing your onion in large nonstick pot (my favorite is All Clad Nonstick, safe and high quality) over medium-high heat, stirring occasionally. When the onion starts to stick, add a tablespoon of water and stir, scraping the bottom of the pot with your heat safe spatula. Repeat this process for 5 minutes until the onion becomes translucent.
Cook garlic: Add the garlic and sauté for another minute, stirring frequently and adding a tablespoon of water when needed.
Add the cauliflower, potatoes, and broth: Bring everything to a boil, then reduce the heat to medium and simmer, covered, for 5-7 minutes until the potatoes and cauliflower can each be pierced easily with a fork. If you need to cook them a bit longer, do so. Remove from heat.
Blend the soup: If you have an immersion blender, add the nutritional yeast if using and carefully blend the soup in the pot until smooth. If you are using a countertop blender, wait 5 minutes for the soup to cool a little and use oven mitts to carefully transfer the hot soup to the blender. If you have a 64 ounce or larger blender container, you *should* be able to fit the soup in one batch (I have a 64 ounce blender and I can). If not, you may have to blend in batches. Add the nutritional yeast and carefully blend until smooth, making sure to place of kitchen towel on top of the blender lid to prevent hot steam from hurting you.
Season and serve: Add the lemon juice to the soup and some dried thyme if you wish. Ladle the cauliflower potato soup into bowls and serve with dissolved miso (whisk 1-2 teaspoons miso paste into 2-3 tablespoons warm water per bowl), fresh cracked black pepper, and fresh or dried thyme.