Mash the boiled potatoes with olive oil and butter, then season with salt and pepper.
Melt butter in a pan and sauté the fennel until soft and golden.
Add white wine and vinegar, simmering until the liquid reduces by half.
Stir in the cream and reduce further until the sauce thickens.
Stir in chives and season to taste.
Heat olive oil in a pan and cook the seasoned sea bass skin-side down for 3 minutes.
Turn the fish and cook for 2 to 3 minutes until done.
Add lemon juice to the fish and remove from heat.
Serve fish on top of potatoes with the sauce poured around.