In a blender, combine onion, garlic and enough water to make a smooth paste. Set aside.
In a large pot, combine potatoes, celery, salt, pepper, onion mixture and juice from clams. Augment with enough water to cover. Bring to a boil, then reduce heat and simmer until potatoes are soft, 20 minutes.
Stir in half-and-half and sugar. Combine melted butter and flour, then whisk into soup. Cook and stir until thickened. Stir in clams and adjust seasonings.