In a large stock pot (large enough to have all of your bigger fish laying flat), spread successively the following ingredients laying flat.
First the thinly sliced onions and the crushed garlic with olive oil, then the potatoes sliced ½ cm (1/5 of an inch), fennel branches, bay leaves, the peeled and thinly sliced tomatoes, saffron and pastis.
Add salt and pepper.
Add the hard flesh fish on top of the above ingredients. On a high heat leave leave the pot 5 to 10 minutes without stirring, but lightly shaking the pot to avoid burning the onions.
Pour the re-heated soup (step #1) over the fish to cover. If there is not enough add a little water. The timing for cooking is now crucial.
When the soup in the pot is boiling, lower the heat to medium (to a light boiling) for 5 minutes.
Then add the tender fleshed fish (they should be covered by the soup as well). Cook for another 5 minutes.
Check if your fish is cooked (white to the bone). If not, put it back in the warm soup for a few extra minutes.
Remove the fish as they are cooked and keep warm.
Remove the potatoes slices and set aside (keep warm).
Filter the soup through a strainer to recuperate the soup only.
On table set the soup, the fish with potatoes and the Rouille.
Some will prefer starting with the soup, adding garlic croutons topped with rouille & grated cheese, and finish with the fish. Others will have their fish & potatoes sauced with the soup using rouille as a mustard.