In a large saucepan, saute the onion and garlic in butter over low heat for about 10 minutes. Pour in the stock and bring to a boil. Add the orange rind and fennel, then reduce heat. Cover and simmer for 45 minutes. Puree and strain, pressing hard. Discard solids, then return soup to the saucepan. Add cream, salt, and pepper, reheat, and serve immediately. Garnish each bowl with a curl of orange zest and a sprinkling of fennel fronds.