A gratin is, essentially, a layered dish cooked in a cream sauce. Typically it’s made with potatoes, less commonly with vegetables. But there is no reason it cannot be made with other ingredients.
Indeed, the base combination of potatoes and a milk or cream sauce is essentially the base for a fish pie and with a little imagination, a gratin can be reworked as a complete meal with fish fillets, mushrooms and prawns (shrimp) added.
Cod, Prawn and Mushroom Gratin
675g (1 1/2 lbs) medium new potatoes (do not peel)
450g (1 lb) cod filet, skinned and cut into 3cm (1 in) pieces
1 bouquet garni
350ml (1 2/5 cups) whole milk
225g (8 oz) button mushrooms
25g (1 oz) plain flour
25g (1 oz) butter
115g (4 oz) cooked prawns, peeled
2 tsp Dijon mustard
1 tbsp fresh tarragon or dill, finely chopped
85g (3 oz) Gruyère cheese, grated
salt and freshly-ground black pepper, to taste
Bring a pan of lightly-salted water to a boil. Add the potatoes and cook for about 25 minutes or until just tender. Drain the potatoes, allow to cool until they can be handled and slice thickly.
In the meantime, poach the cod with the bouquet garni in seasoned milk for 5 minutes. Add the mushrooms to this mixture and cook for 5 minutes more. Drain the fish and mushrooms (reserve the milk).
Place the fish pieces and mushrooms in a shallow baking dish then scatter over the prawns.
Melt the butter in a pan, scatter over the butter and stir to form a smooth roux. Whisk in the milk and heat gently until thickened.
Bring the mixture to a boil then reduce to a simmer and cook for 4 minutes. Discard the bouquet garni then whisk in the mustard, herbs and seasoning. Pour this mixture into dish and stir gently to combine.
Cover with overlapping slices of potatoes then sprinkle over the cheese and place under a pre-heated grill until just golden.