Preparation

• Preheat your oven to 180°C (350°F).
  
  Start by rehydrating the dried porcini mushrooms. Place them in a bowl and cover with hot water. Let them soak for about 20 minutes or until they are soft. Drain and chop them coarsely.
  
  In a large skillet, heat the olive oil and butter over medium heat. Add the chopped onion and minced garlic, sauté until they become translucent.
  
  Add the fresh mushrooms and rehydrated porcini mushrooms to the skillet. Cook them until they release their moisture and start to brown. This should take about 10 minutes. Season with salt and pepper to taste.
  
  In a separate saucepan, heat the heavy cream and milk over low heat. Once it's warm, add half of the grated Parmesan and half of the grated Gruyere cheese. Stir until the cheese is melted and the mixture is smooth. Season with a bit of salt and pepper.
  
  If you want to add a layer of potatoes, arrange the thinly sliced potatoes in a greased baking dish first. Then layer the mushroom mixture on top.
  
  Pour the cheese sauce over the mushrooms and potatoes (if using).
  
  Sprinkle the remaining Parmesan and Gruyere cheese on top of the gratin.
  
  Bake in the preheated oven for about 25-30 minutes or until the top is golden brown and the gratin is bubbling.
  
  Garnish with fresh thyme leaves if desired and serve your delicious gratin aux porcini hot.