Heat your oven to 200°C/Fan 180°C/Gas 6. Unwrap the chicken, remove the giblets and leave it to come up to room temperature - 30 mins should be fine. Fill and boil your kettle.
Meanwhile, pop the dried porcini mushrooms into a small heatproof bowl and cover with boiling water. Leave for 15 mins to soak. Once soaked, drain the mushrooms and rinse well under a cold tap. Shake any excess liquid from the porcini and set them aside.
Peel and dice the carrots. Trim the ends off the celery stick and thinly slice it. Trim the roots and top 3cm off the leek and slice it in half lengthways. Rinse out any grit, then thinly slice the leek. Peel and thinly slice the garlic cloves. Scatter the carrots, celery, leek and garlic into a large, deep roasting tin. Add the soaked porcini. Crumble in the stock cube and add 1 tbsp mustard. Pour in 750ml boiling water and stir together well.
Rub the chicken with 1 tbsp oil and a pinch of salt and pepper. Pop the chicken on top of the vegetables. Pop a lid on the tin or cover with a double layer of foil, scrunched to the sides to seal. Roast for 1 hr.
After 1 hr, take the lid or foil off the tin and roast the chicken for a further 20-30 mins to brown the top. To check the chicken is ready, pierce a thigh at its thickest part. If the juices run clear with no trace of pink, it's ready. Roast for an extra 10 mins if you think needs it.
Rest the chicken on a board, loosely covered with foil, for 30 mins before carving. Keep the mushroom and vegetable broth from the tin. Just before serving, warm it up. Serve the chicken with the warm mushroom broth.