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Move one of the oven racks into the center position so your fish can cook thoroughly. Turn the oven onto 400 °F (204 °C) and let it preheat completely or else the fish won’t have an even cook. Lay the fish flat on its side on the baking pan. If you’re making multiple fish, then leave about 1–2 inches (2.5–5.1 cm) between them so you have space to add aromatics. Fill in the space around the fish with sliced lemons and red onion so they can cook down and infuse flavors into the meat. You don’t need to use lemon and onion if you don’t want to, but it can add a lot of flavor to the dish. Put the baking pan in your oven on the center rack, and leave it alone for at least 15 minutes.  Check the internal temperature of the fish with a meat thermometer to make sure it’s at or above 145 °F (63 °C), or else it’s not safe to eat. Remove the fish from the oven once it’s done cooking and the meat flakes apart easily. The cooking time for each fish will vary depending on its size and thickness. Transfer one of the tilapia along with some of the cooked onions onto a serving plate with a fresh wedge of lemon. You can eat the skin and the meat of the tilapia. Keep leftovers in the fridge for up to 4 days and in the freezer for up to 3 months.

Summary:
Preheat your oven to 400 °F (204 °C). Place the fish in a baking tray with sliced lemons and red onions. Bake the fish uncovered for 15-20 minutes. Serve the tilapia whole while it’s still hot.