Finely zest the oranges, then slice off the peel and white pith. Use a small, sharp knife to slice the orange segments out of the thin papery membranes. Pick the thyme leaves off their sprigs (if they are woody you can push the sprigs through a sieve and the leaves will collect in it).
Add the butter to a small pan and melt over a medium heat. Tip in the orange segments and the zest, thyme leaves, 1 tbsp each honey and mustard and a pinch of salt and pepper. Cook for 8-10 mins, stirring occasionally, till the oranges are soft. Roughly crush the segments with the back of a spoon. Set aside with the lid on to keep warm.
While the sauce 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 8-10 mins till tender. Drain off any excess water 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 (save any soft, feathery fronds). Thinly slice the fennel bulb.
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, till golden and cooked through. Transfer to a warm plate to rest. Loosely cover with foil.
Slide the fennel into the frying pan and fry for 3-4 mins, stirring occasionally, till slightly caramelised. Stir in the sage leaves and cook for a further 1-2 mins.
Spoon the buckwheat onto 2 serving plates. Top with the watercress, partridge breasts and fennel. Serve with the warm orange and thyme sauce.