For the sea bass, preheat the oven to 180C/350F/Gas 4.
Cook the potatoes and fennel bulb in a pan of boiling, salted water for 15 minutes, or until cooked but still firm.
Strain, peel and cut the potatoes and fennel into thick slices. Set aside.
Blanch the pancetta in a pan of boiling water, then drain and refresh in cold water. Drain once more then place on kitchen paper to dry.
Season the sea bass with salt and freshly ground black pepper. Heat a dash of olive oil in a frying pan until hot and cook the fish skin-side down for four minutes, or until the skin starts to be crisp. Turn the fish over and place onto an oven tray skin-side up, then place in the oven for a further six minutes.
Meanwhile, add the potatoes, fennel and pancetta to the same pan you used to cook the fish and fry for a couple of minutes, or until the potatoes are slightly golden-brown.
Add the spring onions and fry for a further two minutes - be careful not to break the potatoes.
For sauce vierge, briefly warm the olive oil in a small saucepan over a low heat for about 30 seconds. Add the shallot and fry for two minutes. Remove from the heat, add the tomato, lime and balsamic vinegar and stir.
At the last minute add the chopped flatleaf parsley and salt and freshly ground black pepper, to taste.
To serve, spoon the ragoût onto the plate, top with the fish then drizzle the sauce vierge over.