Soak porcini in 300ml/1⁄2 pt hot water for 30 mins. Skin chicken and rub the cut side of the lemon over them. Heat oil in a pan large enough to hold chicken in one layer. Add chicken and brown on both sides. Transfer to a casserole.
Heat oven to 180C/fan 160C/gas 4. Melt the butter in a medium pan. When it stops foaming, add onion and 1 tsp salt, then sauté until pale gold.
Meanwhile, remove the porcini from the water and rinse well. Pat dry and chop small. Strain the water through a sieve, stopping before the last spoonful.
Add porcini and pumpkin pieces to onion and sauté for 5 mins. Add fresh mushrooms and cook for 5 mins more, stirring frequently. Stir in the cream, 4-5 tbsp of the porcini water and seasoning if you wish. Simmer uncovered for 10-12 mins until reduced slightly, then pour over the chicken.
Cover and cook in the oven for 30-40 mins, turning the chicken once or twice and adding a few spoonfuls of water if the sauce becomes a little dry.