Preheat the oven to 180C/160C Fan/Gas 4.
To make the marinated fish and seafood, put the monkfish, gurnard and scallops into a bowl and add the garlic, rosemary, lemon zest and olive oil. Season with salt and pepper. Mix carefully, cover and leave to marinate in the fridge for 1 hour.
To make the stock, line a flameproof roasting tin with a sheet of baking paper. Lay the reserved fish parts and bones on the paper and roast for 25 minutes. Turn them over and roast for another 25 minutes. Scrape the fish bones and parts from the paper and return to the tin.
Place the tin over a medium heat on the hob. Add the wine, stirring and scraping to deglaze the tin. Simmer for 5 minutes, then tip everything into a casserole and add water to cover. Bring to the boil and skim off any impurities from the surface. Lower the heat and simmer for 30 minutes.
Meanwhile, to make the stew, heat the olive oil in another large saucepan. Add the onions, garlic, fennel and red peppers. Cook, stirring occasionally, for 5 minutes. Add the chilli flakes, orange zest, saffron, bay leaves and rosemary and cook for 2 minutes. Stir in the tomato purée and cook, stirring frequently, for 5 minutes.
Add the chopped tomatoes and orange juice. Cook, stirring occasionally, for 10 minutes. Pour the fish stock through a sieve into the stew and simmer for 20 minutes. Taste and adjust the seasoning, if necessary.
To make the croûtes, heat a 1cm/½in layer of rapeseed oil in a wide frying pan. When hot, shallow fry the bread slices until golden on both sides. Drain the croûtes on kitchen paper, rub with the cut surface of the garlic and season with salt.
Add the monkfish to the stew and cook for 1 minute. Discard any mussels with broken shells and any that refuse to close when tapped. Add the gurnard, scallops and mussels and cook for another 2 minutes. Finally add the squid and cook for 2 minutes. Discard any mussels that remain closed.
For the spicy anchovy mayonnaise, place all of the ingredients except the oil into a blender or small food processor and blitz for 1 minute. With the motor running, very slowly add the oil through the funnel, making sure all the oil is fully incorporated before adding more. Season the mayonnaise and scrape into a bowl. Cover and refrigerate until needed.
Serve the stew with the croûtes and spicy anchovy mayonnaise on the side.