Preheat oven to 180 degrees C. Line a baking tray with aluminium foil (twice the size of your baking tray). Wash the fish with cold water then dry with paper towels.
Place the fish on the aluminium foil, skin side down. Whisk together the juice of 1/2 of the lemon, olive oil and garlic then drizzle over the fish. Slice the onion and the remaining lemon into rings then lay over the fish.
Fold the aluminium foil into a packet then bake in the preheated oven for about 15 minutes. Remove from oven, open packet then return to oven and cook until fish is flaky and cooked though.