To make the sauce, place the sugar and rhubarb in a medium-sized pan and cook over a low-to-medium heat for five minutes.
Add the red port and bring to the boil for two minutes.
Add the chicken stock and cook until rhubarb resembles a sauce.
Preheat oven to 180C/356F/Gas 4.
To make the rösti, begin by par-boiling the potatoes for about 10-15 minutes. This will help the rösti to cook through more quickly. Cool until you're able to handle them.
Once cooled, using a grater, finely grate the potato. Sprinkle the potato with salt and place in a large tea towel or cloth. Squeeze firmly and discard the moisture that comes from the potatoes through the cloth.
Mix thyme leaves with potato and place in a small blini pan, about 2-3cm/1in in depth.
Using plenty of oil, cook rösti potato on a medium heat until golden brown on one side.
While rösti potato is cooking, constantly push the potato down using a fork.
Turn rösti and continue to colour until golden on both sides. NB: Rösti potatoes can be made in advance and reheated just before serving.
For the duck, on a medium-to-high heat, place duck breasts in a frying pan skin-side down and cook until skin is a light golden brown.
Place duck breasts in the oven for 8-12 minutes while continuing to check from time to time that the skin of the duck does not become too dark.
Remove duck from oven and rest for 4-5 minutes.
In a separate pan, sauté spinach in a little butter until soft.
Using the back of a slotted spoon, squeeze excess moisture out of spinach (discard) and keep spinach warm.
Place spinach and potato on serving plate, slice duck and arrange in a fan shape around the spinach and potato.
Drizzle sauce around the plate and serve.
