Melt butter in a large pot over medium heat.
Add onion, and saute for a minute or two until slightly softened. Add celery, and cook for a couple more minutes to soften the vegetables and develop the flavours. Add garlic, and cook for 30 seconds or so.
Sprinkle flour over vegetables, and stir to incorporate, cooking for about a minute. Slowly add vegetable stock, stirring constantly, followed by the milk. Gradually bring the mixture up to a boil.
Once the broth is boiling, add the carrots, potato, mixed vegetables, salt, and nutritional yeast (if using). Return to a simmer.
Allow the soup to simmer, uncovered, for 35 - 40 minutes, stirring occasionally, until vegetables are tender.
Pulse the finished soup with an immersion blender a few times to break up some of the larger pieces, but leave it fairly chunky. (You can also do this by blending a few cups of the soup in a regular blender, but don't fill it too far, and cover the top with a towel in case the hot liquid sprays out around the lid.)
Add pepper, and taste for seasoning, adjusting if necessary.
You can substitute corn starch for the flour for a gluten-free version by reducing the amount to 3 tablespoons, and whisking it with a bit of cold stock or milk before adding to the pot with the rest of the liquid.