Wash, peel and roughly chop the carrots. Also cut onion into cubes.
Pre-heat oven to 180 degrees.
Place chopped carrots, onions and celery on a baking tray. Drizzle olive oil and sprinkle salt over them. Mix well and roast in a pre-heated oven at 180 degrees for 40-45 minutes. Toss twice in-between.
Take them out of the oven and let them cool slightly.
Once cooled, take them in a mixer/blender jar and blend smooth adding about 2 cups water.
Heat soup in a non-stick pan. Check consistency and add water accordingly. Also add salt, pepper powder and chopped celery to taste. Bring soup to a boil.
In the end, add butter. Mix and remove from flame. Serve hot or warm.