Saute onions in butter about 5 minutes until tender and golden. Add tomatoes, tomato paste, lemon rind, chicken stock, bacon and herbs. Bring to boil, reduce to simmer until tomatoes are tender, about 15 minutes. Remove lemon rind. Cool soup, then puree in blender. Strain puree. Reheat before serving. Garnish with croutons and basil sprigs.