Salt and freshly ground pepper, to taste
Heat the butter and oil together in a large, heavy pot.
Stir in the onion, garlic and celery and cook, stirring, for 10 minutes.
Stir in the carrots and stir for 2 to 3 minutes longer.
Add the stock, tomato purée and potatoes.
Bring to boil and simmer, partially covered, for 20 minutes.
Make Basil Meatballs according to following recipe; add to soup and cook 20 minutes longer.
Season to taste and serve piping hot.
Serves 8 to 10. Basil Meatballs:
Lightly toss together all ingredients.
Form into tiny meatballs and add them to the soup.