In a medium bowl, mix together celery, onions, garlic, seeded tomato (strain and set the liquid aside). Pre-heat oven to 425° F. Place mixture on a sheet pan lined with foil and roast in oven for 30 minutes.
In a separate bowl, mix the strained tomato liquid, olive oil, tomato paste and 1 tbl fresh basil together.
In a Cuisinart, mix together the roasted vegetables, potato and 1 tbl fresh basil. In a heavy quart pot, add the ingredients from the Cuisinart, tomato-vegetable juice, bay leaf, bouillon cubes and lowfat buttermilk. Simmer for 10 minutes. Salt and pepper to taste.
Serve hot. Ladle the soup into individual bowls and garnish with fresh basil.