1: To make the black grape sauce, put the butter into a saucepan and melt it over a medium heat. Add the red onion and sweat gently for approximately 10–15 minutes until soft, stirring from time to time. Pour in the port and red wine, then add the grapes, blackberries and redcurrant jelly. Bring to the boil, turn the heat down and simmer for 25–30 minutes, until the sauce has reduced by half. Remove from the heat.
2: Blitz in a blender or food processor until smooth, pass through a fine sieve, season, and keep in the fridge until needed if not using immediately.
3: To cook the celery, cut the root in half lengthways and trim any brown or dirty bits from it. Pick any leaves from the celery and reserve. Peel any tough strings from the celery heart halves with a vegetable peeler then place the heart in a high-sided frying pan. Pour in the water and dessert wine and add the butter. Season with a generous pinch of salt, place the pan on a medium–high heat and bring to the boil.
4: Turn the heat down to a simmer and cook until the celery heart is soft, the cooking liquid has reduced and the butter and water have emulsified – this should take about 25 minutes. Baste the celery with the cooking liquid, stir in the thyme leaves and keep warm.
5: With a sharp knife, cut a few incisions in the sea bass skin to prevent it from curling up during cooking. Don’t cut too deeply into the flesh. Dust the sea bass flesh with flour and shake off any excess. Season with salt
6: Heat a large, non-stick frying pan over a medium heat and pour in a little vegetable oil. When hot, place the sea bass fillets in the pan, skin-side down, and hold them in place with your hand until you feel that they’ve relaxed and the skin no longer wants to curl up. Cook until the skin is crisp and the fish is cooked 90 per cent of the way through – this should take about 3–4 minutes. If you cook it over a medium heat, you will get a much better result and a crispy skin. If you cook it too high and fast, the skin will burn and cook unevenly.
7: Flip the sea bass over, add the knob of butter to the pan and squeeze in the lemon juice. Baste the fish so it is ready to serve.
8: Warm the grape sauce – you may want to stir in a knob of butter to enrich it. Pour a spoonful of the sauce (you won’t need all of the sauce, but it freezes well so you can save any extra for another time) on to a warmed serving plate and place a sea bass fillet and a piece of celery heart on top. Sprinkle over half the toasted hazelnuts and drizzle on half the hazelnut oil. If you have any leaves left from the celery, garnish the top with them. Repeat with the other fillet then serve immediately.