Melt the butter in a medium sized saucepan and add to that the diced onion, courgette and wild mushrooms and cook over a very low heat for 5-6 minutes until all of these ingredients are softened completely. Mix in the lemon zest, the freshly chopped sage and also the soft white breadcrumbs. Season this mixture lightly. Allow to cool down.
Preheat the oven to 200°C (400°F/Gas 6).
Stuff the cavity of the chicken with the cold stuffing and secure the flap with a cocktail stick. Place the chicken onto a roasting tray. Loosen the skin of the chicken and taking a little additional butter gently massage the butter into the breasts of the chicken. Sprinkle a little bit of salt and cracked black pepper over the skin.
Transfer the chicken to the oven and roast for 15-20 minutes. At this stage reduce the temperature of the oven and cook for a further hour or until the juices run clear out of the chicken. The flesh (particularly on the leg and thigh) should feel tender indicating the bird to be cooked.
For the Red Onion Gravy:
Dice the red onions into relatively small pieces. Sauté the red onions in a medium saucepan with the butter until they are almost fully softened. At this stage you can sprinkle the plain flour over the onions and allow them to become coated. There is no need to take the pot off the direct heat to do this because browning of the flour will only further develop the flavour of the sauce.
Now pour in the red wine and the stock and whisk continuously until the sauce has thickened. One the chicken is roasted add in the pan juices off the chicken and these will add a tremendous flavour and body to the sauce.