Place the prepared mussels in a large wide, dry saucepan, cover and cook over a low heat (there's no need to add any liquid) until they all open.
Remove and discard the empty half shells and throw away any mussels that haven't opened. Arrange the remaining mussels in their half shells in four individual heatproof dishes. Next, in a basin, combine the butter, garlic, lemon juice and parsley, season with salt and pepper and mix well. Place a small dab of the savoury butter on each mussel then put the dishes on the top shelf of the oven and cook for 5-10 minutes or until the mussels are sizzling noisily.
Serve immediately, still sizzling and bubbling.
Pre-heat the oven to gas mark 8, 450ºF (230ºC).
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