Make the marinade first, because the fish will need to sit in the liquid for 30 minutes before it can be sealed in the parchment paper. You can marinate it for up to one day, if you want to.
- 6 (5 1/2-ounce) fresh tuna steaks , butterflied
- 1 cup low-sodium soy sauce
- 1 small fresh jalapeno pepper , seeded and chopped
- 2 tablespoons peeled, grated ginger
- Juice from 3 limes (1/4 cup)
- 1 bunch cilantro , leaves only, reserving 6 sprigs for garnish
- 6 cups fresh, washed, and chopped spinach leaves with tough stems removed
- 1/2 cup sliced oyster mushrooms or red bell pepper strips
- 1/4 teaspoon orange zest
- 3 tablespoons grated fresh ginger
- 3/4 cup thinly sliced red onion
- 1 egg white , slightly beaten
- 1 lemon
- 6 sprigs cilantro (reserved from above)
Cut the parchment paper into 6 squares that measure approximately 13 x 18 inches. Fold each square in half and cut the shape of 1 half of a heart. When you open up the parchment paper, you will have 1 complete heart-shaped piece.
Preheat oven to 350°F. Remove the fish from the refrigerator after 30 minutes, when the fish has soaked up the flavors of the marinade. Toss the chopped spinach, mushrooms or peppers, orange zest, ginger and onions together in a bowl. Open up the heart-shaped parchment and lay 1/6 of the spinach mixture a little off-center on the heart. Arrange the fish on top of the spinach. Drizzle one tablespoon of the leftover marinade over the fish. Repeat for each heart-shaped parchment paper.
Brush the outer edges of each parchment paper with the egg white, using a pastry brush. Fold one edge of the parchment paper over to meet the other edge. Seal the contents inside by making small folds starting at the top, all around the outer edge of the paper, twisting the paper at the end. Put the papillotes on a baking sheet and bake for 15 minutes. When completely cooked, the parchment will puff up with steam.
Using a zester, make 6 grooves in the side of the lemon from top to bottom, then cut the lemon into 6 slices.
Before serving, let the papillotes deflate a bit. If you try to open them immediately after cooking, you will get burned by the steam that has accumulated inside. Unseal the papillote along the outer edge to reveal the food inside. Garnish with cilantro sprigs and lemon slices.