Wash hands with warm, soapy water for 20 seconds.
Preheat oven to 450°F.
Divide fish fillets into 4 portions and place on 4 sheets of aluminum foil (12x18-inches each). Sprinkle with salt and pepper.
Lay lemon rounds over fillets or sprinkle fish with lemon juice.
Evenly divide chopped vegetables between packets, placing on top of fillets.
Wrap each packet by bringing up the sides. Double fold the top and ends to seal. Leave room for heat to circulate.
To bake in the oven, place packets on a baking sheet. Bake at 450°F for 20-30 minutes. Halfway through baking, use a fork to poke small holes in the packet for steam to escape. To bake in an electric skillet, heat skillet to 400°F. Use a cooking rack in the bottom of the skillet. Pour ½ inch of water into the skillet. Place the packets on the rack. Cover and close the steam vent. If you do not have a rack, make your own with aluminum foil. Form foil into a long snake about 1 inch thick. Form into a coil to fit in the skillet. Place the packets on top of the coil. Packets are done when the fish reaches 145°F on the food thermometer. When cooking is done, carefully open packets to allow steam to escape.