Preheat oven to 350°F.
In an oven-proof fry pan over medium heat, heat oil. Add onions and saute, then add apples and increase heat to medium-high. Pour in vinegar, juice and syrup and bring to a boil.
Move from stove to oven, and bake until liquid reduces to lightly thickened syrup, about 45 to 60 minutes.
Remove from oven, extract apple pieces to cool and set syrup aside. Slice cooled apples into matchstick-sized batons. Add back to syrup and set aside.
In a double boiler over simmering water, whisk all ingredients together until mixture becomes thick and frothy, being careful not to scramble the eggs.
Season tenderloin with salt, pepper and rosemary.
In an oven-proof pan over medium-high heat, sear tenderloin.
With the oven still at 350 F, roast tenderloin for 20 to 30 minutes until internal temperature reaches 160°F. Remove from oven and let rest for 5 to 10 minutes.
To serve, slice the pork into 1/4-inch medallions and create an offset stack on plate. Meanwhile, warm braised apples over medium heat, then spoon over pork.
Spoon sabayon over apples and pork. Garnish with truffle slices and, for an extra blast of truffle, drizzle with truffle oil.
- 4 appetizers