Warm butter, cream, garlic, mustard, lemon juice, salt and pepper in a small pan.
Pour the sauce over the fish.
This sauce is not intended to be thick and creamy. The sauce consistency will depend on fish used and whether your fish is fresh or frozen. If you prefer a thicker sauce, you can drain the liquid into a pot after the fish has come out of the oven and heat it on the hob, to thicken.
I would not recommend using frozen fish fillets here because they do give off a lot more water.
You can use dairy milk instead in this recipe but as far as food combinations go, I wouldn't recommend it.
You could use cream instead but I would opt for a non dairy version!