Add the butter to a pan and melt over a medium heat. Tip in the cranberries, thyme, honey, 1 tbsp mustard and a pinch of salt and pepper. Cook for 8-10 mins, stirring occasionally, till the cranberries are soft. Roughly crush the sauce with the back of a spoon. Set aside.
While the cranberry relish cooks, pour the buckwheat into a pan and cover with 300ml boiling water. Put a lid on the pan, bring back to a simmer and cook for 8 mins. Drain if necessary and tip back into the pan. Set aside, with the lid on.
While the buckwheat simmers, trim the dry tips off the fennel. Thinly slice it.
Pour 1 tbsp oil into a frying pan and warm over a medium-high heat. Season the partridge breasts with salt and pepper. Fry for 2-3 mins on each side, until golden and cooked through. Transfer to a plate to rest. Loosely cover with foil.
Slide the fennel into the frying pan and fry for 3-4 mins, stirring occasionally, until slightly caramelised. Stir in the sage leaves and cook for a further 1-2 mins.
Spoon the buckwheat onto plates. Top with the watercress, partridge breasts and fennel. Serve with the cranberry relish.