Preheat oven to 450F.
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.
Lay baby spinach next to the crease, leaving several inches above and below. Sprinkle some of the chopped garlic and season with a little salt and pepper.
Salt and pepper both sides of the fish and place on top of the bed of spinach. Fan the potato slices over the fish starting with white then purple and continue in that pattern until covered. Drizzle 2 tablespoons of vinaigrette and scatter a few thin shavings of black truffles over top.
Seal up the pouch: Fold the parchment over on top of the fish. Starting at the top inner corner, fold and crimp the edges approximately every quarter-inch, until youve created a half-moon. Be sure the folds are well creased to keep the steam from pushing the pouch open in the oven.
Bake for 13-16 minutes or until fish is cooked to your liking. Place the packet directly onto the plates and open carefully. Serve with the remaining lemon truffle vinaigrette (warmed) on the side.