Grab a very large pot (about 6 ½ quarts) with a lid and set aside.

Add the onion and garlic with the oil into the pot and sauté over medium heat for a few minutes. Season with a couple pinches of salt and pepper.

Add the celery, carrots, broccoli, sweet potato, and squash, one by one, as you chop them. Continue to sauté over medium heat, stirring every once and a while so it doesn't stick to the bottom.

Cover the pot with a lid and cook the vegetables for 4 to 5 minutes, reducing heat if necessary.

Remove lid and stir in the broth. Bring the soup to a low boil. Reduce heat to low/medium and cover with lid. Simmer for 10 to 15 minutes, until the squash and potato are fork tender.

Turn off heat and remove lid. Allow the soup to cool slightly for 5 minutes or so. After cooling, carefully scoop the soup into a blender (you'll have to do this in a couple batches most likely) and add in the nutritional yeast and optional cayenne, if using. Carefully blend the mixture with the lid ajar (to allow heat to escape), starting at a low speed and increasing the speed until smooth. Season with salt and pepper, to taste. Alternatively, you can use an immersion blender.

Pour all of the puréed soup back into the original pot. Now, stir in the vinegar or lemon juice, to taste, about 1 teaspoon at a time. I usually add between 1 to 2 teaspoons of white wine vinegar. This helps give the soup flavours some brightness.

Ladle into bowls. Garnish with toasted pepita seeds and croutons.

Transfer leftovers into jars and allow to cool before securing the lid and placing in the fridge. The soup should stay fresh for a week in the fridge. To freeze, add cooled soup into a container or glass jar, leaving 1 inch at the top for expansion. Secure lid and freeze for up to 1 to 2 months.