Makes 2 individual sized tarts.
To make pastry, run flour, oats and margarine together. Add mixed herbs and enough cold water to mix. Roll pastry, and line your small tart tins. Prick pastry before baking, and bake blind for 15 minutes in a 200Of/gas mark 6 oven. After 15 minutes, remove foil and bake for a further 5 minutes until golden brown.
While pastry is cooking, prepare filling by poaching fish in milk. Drain fish, reserving the milk for the sauce, and flake.
To make the sauce, melt margarine and gently sauté onion and leek until soft. Stirl in the flour and mustard. Remove from heat and gradually add all of the milk that the fish was poached in. Bring to the boil, stirring continuously until the sauce thickens. Add the flaked fish and pour the mixture into the prepared pastry cases.
Prepare the crumble topping by melting the margarine, then stirring in the breadcrumbs and oatmeal. Sprinkle onto the tarts and bake in the oven until crisp and golden.