Fish en Papillote with Julienned Vegetables and Truffle Oil

I have several recipes for fish en papillote (that's "butterfly" en Francais due to the shape of the parchment paper), but yesterday I tried a version from Sara Moulton. It was originally published in her book, "Sara Moulton Cooks at Home." At another time, I'd like to test Alton Brown's take on it, with salmon, dry vermouth, and orange wedges.

I cooked dinner for three people, so I adjusted the amounts of the following ingredients.

FISH EN PAPILLOTE WITH JULIENNED VEGETABLES AND TRUFFLE OIL

makes 2 servings

1 1/2 tbs plus 2 tsp unsalted butter
2 carrots, julienned (I julienned baby carrots)
1 large leek, julienned
1 celery rib, julienned
Kosher salt and freshly ground black pepper, to taste
Vegetable oil, for brushing
2 thin fish fillets, no thicker than 1/3 inch, such as pompano or flounder, about 6 ounces each, skin removed (I used flounder)
2 tsp fresh lemon juice
2 tsp white truffle oil (I substituted olive oil)

Melt 1 1/2 tbs of the butter in a large heavy skillet over medium heat. Add the carrot, leek, and celery. Season with salt and pepper. Cook gently, stirring often, until softened, 5 to 7 minutes. Remove from heat and cool.

Preheat the oven to 425 degrees. Cut out a 13 x 15" parchment paper rectangle. Fold in half crosswise. Unfold and brush the whole rectangle lightly with some oil. Lay the fillets on one half of the parchment, skinned side down, and season with salt and pepper. Drain the vegetables of as much liquid as possible and divide among the fillets. Drizzle with the lemon juice. Top each vegetable mound with 1 tsp butter and drizzle each with 1 tsp of the truffle oil.

Fold the other half of the paper over the fillets and, starting with the top, make 1/4" folds all around the perimeters and press to crimp and seal. Close the final crimp with a paper clip. Place the packet on a baking sheet and bake until puffed and lightly browned, 10 to 12 minutes. Transfer to a warmed serving plate and serve at once, preferably while the packet is still puffed.