Melt the vegan butter in a large pot on medium-low heat.
Add in the onions and sauté for five minutes. Add in the thyme and celery and continue cooking for another 5 minutes.
Add in the peeled and cubed potatoes and cook for 5-7 minutes. Toss with the flour and mix until well coated. Cook for 1-2 minutes to cook out the raw flour taste.
Add in the broth, mix and cover the pot. Cook for 20-25 minutes stirring occasionally until potatoes are fork tender.
Remove a cup of the potatoes and set aside. Add in the plant milk and milk. Using an immersion or regular blender, blend the soup until creamy or until desired consistency.
Add back the reserved potatoes and add in the horseradish, mix well. Season with salt & pepper and allow the soup to heat through before serving with chopped parsley and herbed croutons.
For the Herbed Croutons
Toss the cubed bread with the oil and spices. Spread them out in a single layer on a lined baking sheet and bake at 350ºF/180ºC for 8 minutes. Flip and continue baking for another 5-8 minutes. Watch carefully as they can quickly go from golden brown to burnt!