In a pan melt the butter with the sugar.
Add the sea bass fillet and cook for four minutes on each side, or until cooked.
Add the white wine vinegar and remove from the heat.
For the compote, heat the oil in a pan. Add the onion and fry for five minutes, until golden and soft.
Add the sugar, lemon juice, mushrooms and white wine and cook for a further five minutes.
For the caramelised fennel, heat a frying pan and add the oil and sugar.
Add the fennel and cook for three minutes on each side, until caramelised.
To serve, place the compote onto the plate, add the sea bass and place the fennel on top.