Pour 2 to 3 tablespoons of vegetable stock into a large pot and bring to a rolling boil over high heat, keeping more stock close at hand.
Add onions to pot, lower heat to medium-high, and sauté for 5 to 6 minutes until onions become translucent, adding a little bit of stock at a time, as needed, to keep onions from burning and sticking.
Add celery, garlic and zucchini to the onions in the pot and sauté for another 10 to 12 minutes until the zucchini is very soft. Continue to add vegetable stock a little at a time as needed to prevent burning and/or sticking sticking.
Add sliced broccoli, peas and potatoes to the sautéd veggies in the pot and sauté for another 3 or 4 minutes, adding vegetable stock as needed to prevent sticking. Note that the broccoli and peas don't need to be thawed before being added.
Add the remaining stock, bay leaves (if using), thyme and nutritional yeast to the pot. Bring to a boil, then reduce heat and simmer for 20 minutes, until the the potatoes are soft all the way through.
Turn off heat. Remove bay leaves (if using) then without removing the soup from the pot, use an immersion blender to process soup until smooth.**
Add 2 tablespoons of lemon juice, season with salt and pepper to taste. Taste the soup to decide if you want to add more lemon juice and serve.
Optional: garnish each serving with 1 to 3 teaspoons of simple lemon cashew cream, plant-based sour cream or plain unsweetened plant-based yogurt. Makes 6 generous servings.