Arrange the halibut steaks in a dish in one layer.
Mix together the oil, lemon juice, Meaux mustard and salt and pepper to taste and pour over the fish.
Leave to marinate for at least 30 minutes, turning occasionally.
Drain the fish steaks and arrange in a baking dish in one layer.
Melt the butter in a frying pan.
Add the onion and fry until softened.
Stir in the breadcrumbs, cheese and parsley.
Spread this mixture over the fish steaks.
Bake in a preheated moderate oven, 180°C (350°F), Gas Mark 4, for 30 minutes or until the fish is cooked.