heat oil in large dutch oven or soup pot and add leek. saute until softened and fragrant, 7-10 minutes. add garlic, thyme, paprika and cook 2 minutes to release flavors. add celeriac, potatoes, stock, and water. bring to a boil and reduce, simmer 30-40 minutes half-covered, or until vegetables are tender.
stir in lemon juice and, working in batches if needed, blend soup on high to a smooth consistency. thin with stock or water if too thick.
to make croutons preheat oven to 400 degrees. in a small bowl whisk together oil and horseradish. dress bread cubes with oil and roast on a sheet pan for 20 minutes.
serve soup with thyme oil, celery leaves, horseradish croutons.