Place butter and onions is a stock pot on medium heat and saute ~5 minutes, until onion has softened.
Whisk flour and a portion of the chicken broth in a small mixing bowl.
Add¾ of the chopped basil, the chopped tomatoes, the flour/broth mix and the remaining broth, along with the lemon juice and salt to the stock pot.
Bring to a bubbly boil; reduce heat to low and simmer ~5 minutes.
Stir in the cream and half-and-half.
Heat through, but do not boil; serve immediately and garnish with remaining chopped fresh basil.