Bring the sugar, 6 tablespoons of water and orange peel to the boil briefly and allow to cool. Add orange and lemon juice and Campari to the sugar solution. Stir everything well and fill into a metal bowl as wide as possible. Place in the freezer. If a layer of ice forms on the surface, stir everything with a whisk.
Stir repeatedly until the mixture has a smooth consistency. Form 4 large scoops with an ice cream scoop and place in the freezer for another 15 minutes. Arrange orange sorbet on plates. Decorate with kumquat slices and mint