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Melt the butter in a pan.
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Add the flour and keep stirring until you get a stiff ball
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Gradually add in the milk, make sure to keep stirring hard and fast to get out as many lumps as possible. If you find if you can't get them all out, run the sauce through a sieve.
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Melt three quarters of the cheese into the sauce and remove from heat
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Chop the onions and place in a bowl along with both tins of tuna
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Place half the tuna mix in a lasagne or quiche dish, add a layer of lasagne sheets and on top of that half the sauce, repeat once more
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Sprinkle the remaining cheese over the top, place in the oven on gas mark 5/190c for about an hour until lasagne is cooked.