Preheat oven to 180°C (350°F). Grease an 8 in. (20 cm) square glass or ceramic baking dish; set aside. Melt butter in saucepan over medium heat. Cook onion and thyme about 5 min., or until onion is tender. Sprinkle with flour and stir to coat onion. Cook 1 to 2 min. Gradually whisk in milk until sauce is smooth. Mix in mustard. Reduce heat to medium-low. Simmer 5 to 8 min. until slightly thickened. Remove from heat. Stir in cheese until melted.
Pat dry fish fillets with paper or kitchen towels. Place in prepared baking dish, cutting fish to fit if necessary. Bake 15 to 20 min. until fish is just cooked through, starts to flake and internal temperature reaches 70˚C (158˚F). Remove from oven.
Preheat broiler. Pour cheese sauce overtop fish. Sprinkle with breadcrumbs and Parmesan. Broil 1 to 2 min., or until breadcrumbs are golden brown. Sprinkle with chives, if using. Serve immediately.
Substitute thyme with rosemary or dill.
Substitute breadcrumbs with crushed soda cracker crumbs.