For the Cream of Celery Soup:

1. Boil water in a large pot.
2. Add the first four ingredients, lower the heat, and cook for 10-15 minutes, or until the vegetables are soft.
3. Meanwhile, heat the butter in a frying pan, add onions, and sauté on their own for a few minutes. Then, add the mushrooms and garlic and heat for 5 more minutes.
4. Remove from heat and collect the remaining water by pouring the mixture through a strainer.
5. Pour a little olive oil onto the onions, mushrooms, and garlic. Add half the flour and stir. Repeat.
6. Now, pour the celery water onto the mushrooms and whisk until smooth.
7. Pour in the celery mix on top and stir.
8. Slowly add the non-dairy milk while stirring. Stop when you are happy with the consistency. You might want up to 1/2 cup more/less.
9. Add the vegetable bullion powder, oregano, and salt/ pepper.
10. Simmer for 15 minutes.
11. Remove about 1/3 of the soup for the casserole and blend smooth with an immersion blender.

For the Casserole:

1. Combined the potatoes, sour cream, celery soup, half of the butter, and the salt and pepper into a casserole dish.
2. Then, top with the crushed crackers and drizzle the rest of the butter on top.
3. Bake 40 minutes at 350°F.