📋 Instructions

1. 1Heat the oven — Set a rack in the center and heat the oven to 218°C (425°F) for 15 minutes. (A fully heated oven keeps the butter from pooling before the fish sets.) A thermometer clipped inside should read 218°C.
2. 2Line the sheet pan — Line a 45 x 33 cm (18 x 13 in) rimmed sheet pan with parchment. (Parchment stops the lemon from welding to the metal.) The paper should lie flat with no curls at the corners.
3. 3Dry the fillets — Pat all 6 fillets dry with paper towel on both sides. (Surface moisture steams the fish instead of roasting it.) The fish should feel tacky, not wet, under your fingers.
4. 4Season the fish — Lay the fillets on the pan, tuck any thin ends under, and sprinkle with 8 g (1½ tsp) salt and 2 g (½ tsp) pepper. (Even thickness means even cooking.) The surface should look evenly dusted, with no bare patches.
5. 5Build the topping — Stir 45 g (3 tbsp) melted butter with 30 g (2 tbsp) olive oil, 12 g (4 cloves) minced garlic and 6 g (1 tbsp) thyme. (The oil raises the smoke point so the garlic sweetens instead of burning.) The mixture should smell sharp and raw.
6. 6Coat and top — Spoon the butter mixture over each fillet, then lay one lemon round on each. (The lemon shields the garlic from direct heat.) Every fillet should be glossy and covered.
7. 7Bake — Bake at 218°C (425°F) for 12 to 14 minutes. (Thin fillets finish at 12; thick center cuts need 14.) The butter should bubble at the pan edges and the fish should flake at 63°C (145°F) internal.
8. 8Finish and rest — Squeeze the juice of 1 lemon (45 g / 3 tbsp) over the pan and scatter 15 g (¼ cup) parsley. (Acid at the end stays bright; acid at the start fades.) Rest 3 minutes, then serve with the pan juices.