In a medium size bowl, combine the mustard, orange zest, 1 1/2 tablespoons lemon juice, salt, pepper, butter, and 3 tablespoons of chives; mix well to blend.
Preheat oven to 425.
In a lightly buttered baking dish, place the fillets skin side down. Brush each fillet with the remaining lemon juice and season with salt and pepper. Spread about 2-2 1/2 tablespoons of butter mixture over each fillet to coat evenly.
Bake for about 12 minutes or until done. To check for doneness, insert a thin-bladed knife into the thickest part of the fillet. The flesh should flake and separate easily and there should be no trace of translucence. The fish should be opaque and flaky. Transfer the fish to a warm platter and brush any remaining mustard chive butter over the fish while it is hot. Sprinkle with the remaining chives to garnish. Cut each fillet in half to serve.
Cooking Times For Fish: As a rule of thumb, you can expect fish to take about 12 minutes per inch to cook through in a preheated 425 oven.