Trim the duck breasts to remove any excess fat and score the skin with a diamond pattern.
To make the sauce, put the sugar and vinegar into a saucepan, bring to the boil and reduce until it begins to caramelise. Add the orange juice, reduce down to one third and add the chicken stock. Leave to simmer.
Set the oven to 400°F Season the duck breast with salt and freshly ground black pepper. Heat a frying pan over a high heat and add the duck breasts, skin side down. Cook until the skin is golden, turn the breasts over and cook for a further 30 seconds. Transfer to the oven and cook for 4-5 minutes, depending on size. When the duck is cooked, remove it from the oven and leave to rest - skin side up.
Heat the butter and add the garlic and spinach. Cook briefly until the spinach has wilted and then season with salt and freshly ground black pepper.
Add the orange zest to the orange sauce and simmer for a minute or so.
To serve, place a portion of drained spinach on each plate, cut the duck breast skin into slices and fan evenly across the spinach. Put the orange segments into the sauce and gently poach for 15 seconds. Place the orange segments around the duck and pour the sauce around the plate.