Preheat the oven to 400°F.

Season the cod fillets on both sides with salt and pepper.

Layout four pieces of aluminum foil, each about three times the size of a fillet. Place a fillet in the center of each piece of foil. Divide the butter, thyme, and lemon juice among the four fillets. Fold over the sides of each foil sheet to form a pouch, sealing the fillet inside. Place the four foil pouches on a rimmed baking sheet.

Bake for 20 minutes. Open the pouches carefully to allow the steam to escape.