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1
Preheat oven to 180C (350F).
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2
Spread the butter on both sides of the toasted bread, cut the slices in half diagonally, and set aside.
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3
Heat a medium sized, heavy frying pan over a moderate heat.
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4
Add the beef, onion, celery, prepared mustard and 1/2 tsp of the salt.
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5
Cook, stirring occasionally, until the beef has browned - around 5 minutes.
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6
In a greased shallow casserole dish, place alternate layers of toast, beef mixture, then cheese.
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7
In a small bowl, beat the egg, then stir in the milk, dry mustard and the remaining salt.
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8
Pour this mix over the casserole (that should have the last cheese layer at the top).
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9
Sprinkle with the paprika, bake for 25 minutes, and serve.
