Put the
onion,
garlic,
mushrooms, tomatoes, tomato puree and oregano in a
saucepan.
Bring to the boil, reduce the heat and
simmer for 15 minutes until pulpy. Stir in the fish
and cook for a further 5 minutes. Season to taste.
Meanwhile, blend the flour, butter and a little of
the milk in a saucepan. Stir in the remaining milk.
Bring to the boil and cook for 2 minutes, stirring
until thickened.
Season to
taste
and add half the cheese. Spoon a thin layer of the cheese sauce in the
base of a 2.25 liters/4 pts/10 cups ovenproof
dish.
Cover with a layer of lasagne, breaking it to
fit. Cover with a third of the fish sauce, then a
little more cheese sauce.
Repeat the layers twice
more, making sure you have plenty of cheese sauce
for the top.
Sprinkle with the
remaining cheese and bake at 190°C/375°F/gas
mark 5 for about 35-40 minutes until the pasta feels
tender when a knife is inserted down through the
centre.