Traditional papillote takes time and requires origami-like folding. Here, we use foil to make a no-mess pouch; the fish becomes infused with the flavors of tomato, capers, garlic, and lemon.
04Spoon hot tomato mixture over fish, then cover with another sheet of foil, tenting it slightly over fish, and crimp edges together tightly to seal.
05Bake until fish is just cooked through, 12 to 15 minutes (depending on thickness of fish); check by removing from oven and carefully lifting up a corner of top sheet of foil, pulling up sides of bottom sheet to keep liquid from running out. If fish is not cooked through, reseal foil and continue to bake, checking every 3 minutes.
06Transfer fillets with lemon slices to plates using a spatula (be careful not to tear foil underneath) and spoon tomatoes and juices over top. Serve immediately, discarding thyme before eating.