Melt 15g butter in a frypan and saute mushrooms until golden brown and quite dry. Leave aside to cool. Mix shallots and garlic in with the mushrooms, add a squeeze of lemon juice and sage leaves. Add the remaining butter and mash together. Let it set in the fridge for a while until the butter is firm.

Gently lift the skin off the chicken creating a cavity and push in the cold mushroom butter. Work it around under the skin.  

Season the outside of the skin with salt and pepper. 

Roast for 40 minutes in a hot oven, basting every 10 minutes.