In a large pot or Dutch oven (about 6 quarts in volume is good), heat the vegetable oil. When the oil is heated, add the celery, onion, carrots and garlic to the Dutch oven. Cook until the veggies are translucent.
Add the diced potatoes, vegetable stock and salt. Bring to a boil, then turn down to a simmer and cook until the potatoes are fork-tender, about 20-25 minutes.
Ladle about 1/2 cup of the broth out into a heat safe bowl, and use a fork or a whisk to stir in the miso. Add the miso back to the pot. Turn off the heat, and very carefully use a stick blender to blend the soup to be about 2/3 smooth and 1/3 chunks - or if you prefer another texture, go for it!
Finally, stir in the lemon juice, milk and olive oil. You can hold this over low heat for quite a while, stirring occasionally. Or, it's great for freezing for work lunches. I like to top mine with chives and bacon.