I learned the technique of cooking fish en papillote (literal translation: in foil; figurative translation: in some sort of packet) from the illustrious Queen Ina Garten's cookbook, Make It Ahead.
Her Herb Roasted Fish is one of the easiest and versatile recipes in the book. It appears to be a later incarnation of a similar recipe available online here. You cook the fish inside a packet made of parchment paper and the result is incredibly moist and flavorful fish - even if you use a less-flavorful (but very affordable) fish like tilapia.
1. Begin with fish filets, divided into individual servings.
2. Place each serving on a sheet of parchment paper. Drizzle with olive oil and lemon juice, season with salt and pepper, and place a spring of thyme and a few olives around the fish.
3. Paint an egg wash around the edges of the parchment, and seal it into a packet by folding the edges over one another. Bake at 400 degrees for 13-15 minutes, depending on how thick your filets are.
The end result is super moist, flaky fish. It's both rich and light at the same time and seems quite decadent, despite how easy it is to make.
This recipe is also extremely versatile - you can experiment with different seasonings, types of fish, and adding vegetables.