Prepare the Fonduta
1. Very slowly reduce cream to ¾ its original volume and thickened.
2. In the meantime, grate fontina cheese on the large side of a box grater.
3. Gradually begin to whisk cheese into the reduced cream. Continue whisking until all the cheese is melted and dissolved into the cream.
4. Whisk in truffle oil and season with salt and pepper. Set aside in a warm place.
Prepare the mushrooms
5. Preheat oven to 450 degrees.
6. Peel the outer layer from mushrooms and cut them in half lengthwise. Score the inner side of mushrooms with a knife in a crisscross pattern.
7. Heat a large sauté pan to medium for about a minute and add olive oil.
8. Place mushrooms in pan with scored side down and slowly cook until they begin to turn golden brown.
9. Add butter, thyme, salt and pepper to taste.
10. Transfer to oven and cook until mushrooms are tender: 8-10 minutes.
11. Spoon fonduta onto a plate and place mushrooms on top.
12. Garnish with mache and slices of preserved truffle.