Heat your oven to 230°C/Fan 210°C/Gas 8. Pop the porcini in a pan (or a jug) with 250 ml boiling water. Leave the butter out to soften.
Remove the chicken from its wrapping. Remove the giblets – you can use them to make your own chicken stock. Pat dry. Season the chicken all over with salt and pepper. Set in a roasting tin. Lift out the porcini from the pan. Pour the soaking liquid around the bird but not over it. Finely chop the soaked porcini, sage leaves and garlic cloves (peel first). Fold through the softened butter. Season to taste.
Carefully lift the chicken skin and tuck the butter in over the breasts. Dust a good pinch of salt and pepper over the chicken. Slide in the centre of the oven.
Cook for 1 hr — no peeking as this will let all the heat out. Take your chicken out of the oven. Let it rest for 30 mins to 1 hr before carving.
Use the roasting tin juices as gravy, or for a richer, more traditional gravy, reduce down with a glass of white wine. To thicken, spoon 4 tbsp roasting tin juices into a dish. Whisk with 1-2 tbsp plain white flour, or enough to make a smooth paste. Whisk this back into your gravy. Gently simmer till it’s as thick as you’d like it. Serve with slices of roast chicken.