01 - Preheat the oven to 400°F. Lightly grease a baking dish large enough to hold the fillets in a single layer.
02 - Pat the halibut fillets dry with paper towels. Brush both sides with olive oil, then season generously with salt and black pepper.
03 - Place the halibut fillets evenly spaced in the prepared baking dish.
04 - Combine melted butter, minced garlic, lemon juice, lemon zest, and chopped parsley in a small bowl; whisk until integrated.
05 - Pour the lemon butter sauce evenly over each halibut fillet and top with a lemon slice.
06 - Bake for 15 to 20 minutes until the fish flakes easily and is opaque throughout, reaching an internal temperature of 140°F.
07 - Remove from oven, spoon pan juices over the fillets, and serve immediately.