Heat the oil in a large saucepan set over low heat. Add the onion and celery and cook, stirring often, for about 5 minutes.
Add the cauliflower and diced potato and cook for one minute, stirring often. Sprinkle the sage, mustard, and celery seed over the vegetables, stir until evenly distributed. Cook for approximately 20 seconds, or until fragrant.
Pour in broth and bring to a simmer over medium-high heat. Cover, reduce heat to low, and simmer until the potato and cauliflower are soft when pierced with a fork, about 15 minutes.
Working in batches, puree the soup in a large blender, scraping down the sides as necessary. Return the puree to the saucepan, set over medium heat, and cook just until heated through, about 1 minute. Season with salt and pepper, to taste.