Preheat oven to 450ºF.
For the vinaigrette, Place lemon juice, salt and pepper in a medium-size mixing bowl and whisk in the olive, canola and truffle oils. Set aside.
In a small bowl, whisk together melted butter and oil. Brush the bottom of a cast-iron skillet with some of the butter mixture. Place thinly sliced potatoes (white and purple) into the skillet and brown both sides over medium heat. Brush the potato slices with the butter mixture and season with salt/pepper. Set aside.
Tear off a piece of parchment paper approximately 18 inches long, fold it in half, then unfold and place it on a baking sheet.