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Heat oven to 200C/180C fan/gas 6. Put the bread on a large baking sheet, drizzle over 1 tbsp oil and bake for 10 mins until golden. Set aside.
Meanwhile, heat the rest of the oil in a large flameproof casserole dish over a medium heat. Add the onion and cook until softened for about 10 mins, then add the garlic and chilli flakes and stir for 1 min. Tip in the tomatoes and fish fillets. Cover and simmer for 10 mins until the fish is nearly cooked, then uncover.
Tip in the butter beans, season well, then cook until everything is hot. Serve scattered with the croutons, parsley and lemon.