Heat half the olive oil in a frying pan and fry the chopped onion with some black pepper until golden.
Season the perch with salt and freshly ground black pepper and give the fillets a light coating of flour.
Heat the remaining oil in a separate frying pan and fry the fish until golden.
Add the passata, capers, garlic and fish stock to the onions, then place the fish in the sauce and cook, uncovered, over a low heat for 15 minutes. Add a little more stock if it starts to dry out.
Season with salt and freshly ground black pepper to taste, garnish with fresh parsley and serve.