In a heavy saucepan melt the butter over low heat. Add the onions, cover and cook until soft, about 15 minutes. Add the garlic and cook 2 to 3 minutes.
Add the tomatoes, stock, lime juice, and sugar. Bring to a boil,reduce the heat and simmer, covered for 15 minutes.
Add the basil and orange zest.
Transfer the soup to the bowl of a food processor and puree.
Cool. Ladle into two 10 cup freezer containers. allow 1 inch of headspace.
Chill in the refrigerator, label and freeze for up to 3 months.