For the fish stew, heat 2 tablespoons of the oil in a wide, heavy-based saucepan over a medium heat. Add the fish trimmings (or steaks) and the small prawns and season, to taste, with salt and freshly ground black pepper. Fry for 8-10 minutes, or until golden-brown all over, then remove from the pan and set aside on a plate.
Add 2 more tablespoons of oil to the same saucepan and increase the heat to high. Fry the fennel, shallots, carrot and celery for 8-10 minutes, stirring occasionally, until they begin to brown.
Stir in the tomato purée and continue to cook for 1-2 minutes. Add the chopped tomatoes, star anise, saffron and cayenne pepper and cook for a further 3-4 minutes.
Pour in the white wine and pastis and bring to the boil, scraping up any burned bits from the bottom of the pan with a wooden spoon to deglaze. Continue to boil until the liquid has reduced to a thick syrup.
Add the stocks and return the mixture to the boil. Continue to boil until the volume of liquid has reduced by one-third, about 10-15 minutes.
Remove the pan from the heat and set aside to cool slightly. Ladle the liquid into a food processor and blend to a smooth purée in batches, taking care to cover and hold down the lid of the food processor with a tea towel to avoid scalding yourself.
Strain the mixture through a fine sieve into a clean pan, then reheat over a medium heat.
Season the pieces of sea bass, cod and halibut on both sides with salt and freshly ground black pepper.
Heat the remaining oil in a large frying pan over a medium heat. Add the fish pieces, in batches if necessary, and fry for 1-2 minutes on each side, or until golden-brown on both sides. Add them to the strained stew.
Add the cockles to the strained stew and cook until the shells open (discard any cockles whose shells do not open during cooking). Add the cooked prawns and stir until heated through. Squeeze over a little lemon juice, then season, to taste, with salt and freshly ground black pepper. Stir in the reserved fried fish trimmings (or steaks) and the cooked small prawns.
For the garlic toast, preheat the grill to its highest setting.
Blanch the garlic cloves in a pan of boiling water for 20 seconds. Drain well, then crush the cloves to a fine paste.
In a bowl, mix the crushed garlic with the softened butter. Spread the garlic butter onto the baguette slices.
Arrange the baguette slices on a baking tray and grill for 1-2 minutes, or until lightly toasted.
For the aïoli, blend the garlic, egg yolks and saffron strands to a paste in a food processor. Pour both oils into a jug, mix the together and, with the motor running, gradually add them to the aïoli in a thin stream until all of the oil has been incorporated and it has thickened to the consistency of mayonnaise. Add the vinegar a few drops at a time, to taste. Season, to taste, with salt and freshly ground black pepper.
To serve, ladle the fish stew into serving bowls. Dollop a teaspoonful of the aïoli on top of each garlic toast and float 2 or 3 toasts on top of each portion. Serve the remaining aïoli in dipping bowls alongside.