Preheat the oven to 200C/400F/Gas 6.
For the rosti, boil the potatoes in their skins for seven minutes, then drain and allow to cool enough so that you can handle them without burning your hands.
Peel the potatoes and coarsely grate the flesh into a bowl. Add the onion, garlic and season well with salt and freshly ground black pepper.
Divide the potato mixture into four and mould each into a patty shape, then flatten slightly on a clean flat surface.
Place onto a clean tray and transfer to the fridge to cool for 20 minutes.
Heat the oil in a large frying pan over a medium heat and add the rosti, pressing them into the pan to flatten.
Fry the rosti for 4-5 minutes on each side, until crisp and golden all over, then remove and drain on kitchen paper.
For the salsa, place a clean frying pan over a medium heat and fry the bacon lardons until crisp and golden-brown all over.
Add the chilli, tomatoes and coriander and stir well, then transfer to a clean bowl.
Add the avocado and season, to taste, with salt and freshly ground black pepper. Drizzle with the olive oil and stir gently to combine.
For the fish, season the fillets well with salt and freshly ground black pepper.
Heat the oil in a large ovenproof frying pan over a medium heat. Add the sea bass fillets, skin-side down, and fry for two minutes.
Turn the fillets over and transfer to the oven to roast for 2-3 minutes, or until just cooked through.
To serve, place the rosti onto warmed plates and place a sea bass fillet on top of each. Spoon some of the salsa over the top of the fillets and serve any extra in a bowl alongside.