1) Take a bowl and season the halibut with salt and pepper.
2) Take a baking dish and heat butter for 30 seconds until it melts completely.
3) Add parsley, garlic, onion, lemon juice and mustard and mix properly to prepare a sauce.
4) Cut the halibut into four equal portions before putting it in the sauce.
5) Top it with the sauce by spooning it over before sprinkling cayenne pepper and mace.
6) Cover and cook for 5 minutes.