Cut the potatoes and the celeriac into medium-sized cubes (about 1 inch). Cut the carrots into half lengthwise and slice them. Slice the leek into rings.
Heat some oil in a large pot and add the vegetables. Cook for 5 minutes on high heat. Stir a few times.
Add the vegetable broth. Cover with a lid and bring to a boil.
Let it simmer for 15 minutes or until the vegetables are tender.
Scoop out about 1/4 of the soup with a ladle.
Use an immersion blender or a regular blender or food processor to puree it until smooth.
Transfer back to the pot. Add the freshly chopped parsley and season with salt and pepper.
Serve with roasted rustic bread.
Notes
Make sure the cut vegetables all roughly have the same size. This way they will have the same cooking time.
If you want your soup creamier, you could puree up to 1/2 of the soup. If you want you could also add a bit of vegan cream.
If you can't find celeriac, you could replace it with parsley root, parsnips, or celery. You could also use more potatoes or carrots instead.