For the pesto, blend all the pesto ingredients in a food processor to a rough paste. Season, to taste, with salt and freshly ground black pepper.
For the tomato sauce, heat the tomato purée in saucepan until it colours. Stir in the stock, bring the mixture to a simmer and continue to simmer until the mixture is well combined and has thickened. Season, to taste, with salt and freshly ground black pepper.
For the sea bass, heat the olive oil in a frying pan over a medium heat. Fry the sea bass, skin-side down, for 3-4 minutes. Season, to taste, with salt and freshly ground black pepper. Turn the sea bass fillet over and fry for a further 3-4 minutes, or until the skin is crisp and golden-brown and the fish is cooked through. Remove from the pan and set aside to rest on a serving plate.
For the leeks, bring all the ingredients to the boil in a saucepan. Reduce the heat until the mixture is simmering and poach the leeks for 8-10 minutes, or until tender. Drain the leeks then season, to taste, with salt and freshly ground black pepper.
To serve, spoon the tomato sauce over the fried sea bass. Serve the poached leeks alongside with a dollop of pesto.