In a saucepan or skillet, melt 2 T. of butter, add the flour and blend with a fork or whisk. Slowly add the milk while cooking over low heat, until the mixture thickens. Add the mustard, herbal seasoning and pepper.
In another saucepan or skillet, heat the oil and cook the onion slices until golden brown and transparent.
Slip the halibut into the boiling water and simmer for 3 minutes. Drain and set aside.
In a shallow baking dish, spread the cooked onions; place the halibut on top of the onions. Pour the mustard sauce over the fish.
Bake at 425° for about 20 minutes or until fish flakes easily with a fork.
