1. Heat oil and saute celery, onion and garlic in heavy-bottomed pot until translucent. Add garlic clove, tomatoes and water and bring to a boil. Reduce heat to low and simmer, covered, about 30 minutes.
2. Strain into a bowl (or use a food mill). Rinse and dry soup pot and return to burner. Melt butter over low heat and add flour; cook until just bubbling, being careful not to burn the roux.
3. Return tomato mixture to pot, add basil, and cook over medium-low heat until soup has thickened a bit (about 10 minutes). Season to taste with salt and pepper.
