In a large pot, over medium heat, add a splash of oil. Sweat the fennel until translucent. Combine 750 millilitres of cranberries, the orange juice, orange zest and the sugar and add to the fennel. Over medium heat, bring the mixture to a boil. Continue to simmer until excess liquid has evaporated and the berries have burst. Remove from heat when the desired consistency is achieved and stir in the remaining berries.