Heat the oven to 180°C/Gas 4. To prepare the partridges, with a small, sharp knife, cut down either side of the wishbone and remove it. Wrap the bacon rashers around the back of the partridges and tie with string (but not too tightly).
Heat a heavy-based saucepan over a medium-low heat and add a drizzle of olive oil. Add the bacon lardons and cook gently for 2–3 minutes. Add the shallots and carrots and sweat gently for 2–3 minutes, then add the garlic and bouquet garni.
Now add the lentils to the pan and stir to mix with the shallots, carrots and bacon. Season with a little salt and pour on the chicken stock to cover. Bring to a simmer, lower the heat and cook gently for 35–40 minutes, stirring occasionally.
Heat a non-stick ovenproof frying pan over a medium-high heat and add a drizzle of olive oil. Season the partridges all over with salt and pepper and place in the pan. Turn the birds as necessary over the heat for 3–4 minutes until they start to take on a lovely golden colour. Add half of the butter, allow it to melt and foam, then spoon over the birds to baste them.
Once the birds are coloured all over, put the pan into the oven. Roast for 8–10 minutes until the partridges are just cooked. Transfer the birds to a warm plate and leave to rest for 5 minutes.
Add a drizzle of oil to the frying pan and place over a medium-high heat. Add the girolles, season with a little salt and sauté for 1–2 minutes until they release their liquid; drain this off, then sauté the girolles for 30 seconds or so until golden and tender. To finish the lentils, add the sherry vinegar and remaining butter.
Remove the bacon from the birds. Cut off the legs, then, using a sharp knife, take the breasts of the carcasses. Remove the skin from the legs and breasts.
Divide the lentils and girolles between warm serving bowls and arrange the partridge breasts and legs on top. Drizzle over the resting juices from the tray. Garnish with watercress and serve.