Divide the butter between 2 frying pans and set both over a medium heat.
To one, add the shallot and sage leaves and fry until both are crisp, then transfer to a plate lined with kitchen paper. At the same time, to the other pan 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 crisp, turn over and cook for a further 3 minutes.
Serve the fish sprinkled with the shallots and sage leaves, and accompanied by the mushrooms.