Heat oil in a soup pot or Dutch oven over medium heat. Add onion, 1/2 tsp salt, and a grind of pepper and sauté for 5 minutes, or until softened. Add garlic and cook for another minute.
Add carrots, broccoli, cauliflower, and potato and sauté for another 3-5 minutes. Add flour and stir well to combine.
Add broth, milk, nutritional yeast and another 1/2 tsp salt. Bring to a boil and then reduce heat to low and simmer, covered, for 15-20 minutes or until veggies are tender. Remove from heat and stir in lemon juice. Season with salt/lemon juice to taste until the flavors really pop.
Use an immersion blender to blend half the soup, leaving some chunky for texture. You can also blend half in a blender, being careful as the soup will be very hot!