For the sea bass, remove the skin and season the fish with salt and freshly ground black pepper.
Wrap each sea bass portion in two of the blanched spinach or lettuce leaves; then wrap each parcel in heat-proof cling film.
Half-fill a steamer with water.
Bring to the boil; then lower the heat so that the water barely trembles.
Place the steaming rack on top of the steamer, arrange the sea bass parcels inside, cover and steam for 8-10 minutes, or until cooked through. To test whether the fish is done, insert a trussing needle or knife tip into the centre for 10 seconds; it should penetrate easily and feel hot when you remove it and place it on your inner arm. Keep the fish warm.
For the leek coulis, halve the leeks lengthways, wash thoroughly in cold water and slice finely.
Blanch the leeks in a pan of boiling water for two minutes, then refresh in a bowl of cold water and drain well, pressing the leeks to dry them.
Melt the butter in a saucepan, add the leeks and fry them over a low heat for 10 minutes, or until softened.
Add the stock, then the saffron, and cook over a medium heat for 10 minutes. Add the cream and simmer for five minutes.
Cool the mixture slightly, then tip into a blender or food processor and blend for one minute. Strain through a metal sieve and season with salt and freshly ground black pepper.
Keep the coulis warm if it is to be served soon, otherwise reheat gently just before serving. Stir in the dill at the very last moment.
To serve, carefully peel the cling film off the fish parcels. Place one on each of four plates and brush with clarified butter.
Pile some of the deep-fried leek on top of each parcel and pour the leek coulis around the edge. Serve at once.