Heat the oven to 180C/fan 160C/gas 4. Melt a knob of butter in a 26cm ovenproof frying pan over a medium heat and season the partridges well. Fry the partridges for 1 minute, breast-side down, until nicely browned, then remove to a plate.
Remove the pan from the heat and layer in the potato and onion slices, seasoning with the thyme leaves, black pepper, some salt and the parmesan between each layer and finishing with a layer of potatoes and a sprinkle each of thyme leaves and parmesan. Pour over the chicken stock, cover the pan with foil and roast for 1 hour 20 minutes or until the potatoes are tender when pierced with the tip of a knife.
When the partridges have cooled, arrange the bacon rashers over the partridge breasts in a lattice pattern. When the potatoes are ready, remove the foil (keep this to one side) and nestle the partridges on top. Roast for 35-40 minutes or until the partridges are cooked through and the bacon is starting to crisp. Cover with the reserved foil and leave to rest for 5 minutes, then serve the partridges alongside the potatoes and onions.