Put the mackerel fillets in a single layer in a large ovenproof baking dish.
Make the mustard sauce. Melt 1 1/2 tablespoons of the butter with the oil in a pan. Stir in the flour and cook stirring constantly for 2 minutes. Gradually stir in the milk a little at a time. Cook stirring constantly for about 10 minutes until thickened. Remove the pan from the heat stir, in the mustard and lemon juice and season to taste with salt.
Pour the sauce over the mackerel fillets sprinkle with the parsley and dot with the remaining butter.
Bake for 10-15 minutes then serve straight from the dish.
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