Preheat the oven to 180C/350F/Gas 4.
Place the fish in large saucepan. Pour over the milk and add the mace and bay leaves. Bring the milk to just under a boil and poach the fish for 5-6 minutes.
Drain the fish, reserving the milk and discarding the bay leaves. Set the fish aside.
To make the bechamel sauce, melt the butter in a small saucepan. Add the flour and stir until well combined.
Gradually add the reserved poaching milk, whisking all the time until the sauce is thick and smooth. Alternatively, add all the milk in one go and use a hand whisk to make it smooth.
Stir in the egg yolks and the lemon juice. Add the poached fish, flaking any big pieces into smaller chunks, then pour the mixture into a medium casserole dish.
Whisk the reserved egg whites until stiff. Carefully fold in the grated cheese. Spread the egg white topping over the fish. Decorate with the cherry tomatoes then bake for 20-25 minutes, or until golden.
Serve with steamed green vegetables.
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