Preheat the oven to 400 degrees F.
Pat the fish fillets dry with paper towels, then place on a sheet of parchment paper large enough to fully wrap the fish. Top the fillets with the butter, oil, lemon juice, thyme and rosemary, and sprinkle with some salt and pepper.
To fold en papillote do as follows: Place the two longer edges of the paper together, make a 1/2-inch fold and then fold again, repeating until you have reached the fish fillet. Repeat with the short edges.
Place in the oven and cook until the flesh flakes apart easily with a knife, 10 to 12 minutes--beware of the hot steam when you cut open the parcel.
Serve the fish as is or over steamed rice or baby spinach. Dress with extra lemon juice and a little parsley.