Preheat the oven to 425 degrees F (175 degrees C).
Stir together panko bread crumbs and melted butter in a bowl until well combined. Set aside.
Pour enough chicken stock into an oven-proof skillet to cover the bottom; arrange halibut fillets in the skillet.
Drizzle lemon juice over fillets; season with salt and pepper. Spread a thin layer of Dijon mustard over each fillet; press bread crumb mixture into mustard.
Bake fish in the skillet in the preheated oven until fillets flake easily with a fork and topping is browned, about 12 - 14 minutes.
