Gratin Of Potatoes And Porcini Mushrooms In Cream

• 2 shallots
• 1 pepper (freshly ground)
• 1 day
• 1,700 gr gratin dish
• 575 gr baking potatoes
• 160 ml milk
• 160 ml whipping cream
• 120 ml water (hot)
• 90 gr salt and pepper
• 45 ml whipping cream