Prepare the fish by cutting off the gills and external hairy bits and discarding. Keep the fish in salt water.
Put the small rock fish (girolles, baby crabs, stone fish) into a large saucepan of cold water on high heat. Add the thyme, rosemary and bay leaves. Bring slowly to a boil, then simmer for 30 minutes.
For the Soup:
Heat 2 tablespoons of olive oil in a large heavy-bottomed saucepan. Cook the chopped onions until translucent. Pour in the tomatoes and cook for 5 minutes, then add the sliced potatoes. Place the pieces of John Dory and the monkfish on top.
Strain the rock fish soup through a strainer on top of the fish and boil for 5 minutes.
Place the rascasse, scorpion fish, and red mullet on top. Add a pinch of saffron and watch soup turn a rich yellow color. Finally add the lobster halves and cook for a maximum of 5 minutes.
Serve the big fish on a serving plate and the soup in a casserole. Serve the pieces of fish on the plate with potato slices and the Croutons with Rouille and pour the soup over the top. Eat immediately.
If you cannot find the above fish ask your fishmonger for something similar.