Divide the butter between 2 frying pans and set both over a medium heat. In one add the shallot and sage leaves and fry both until crispy, then remove to a plate lined with kitchen paper. In the second add the mushrooms and cook for 10 minutes until golden brown.
Add the sea bass fillet to the first pan, skin side down and cook for 3 minutes until the skin is crispy, turn over and cook for a further 3 minutes.
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