We first make the base for the soup that cooks for about an hour. The fish is added after that, just until it is cooked through.
In a large 6 qt. saucepan, heat the olive oil over medium heat. Add the onions, fennel and garlic. Cook slowly for about 5 minutes, stirring. Add the saffron and stir. Chop the tomatoes into a chunky paste and add to the pot with the tomato juice from the can. Add the white wine, stock, salt, and pastiche. If you are adding small bits of fish for flavor, add it now. Cut the potatoes into 3/4" size chunks and add to the pot.
Bring the pot to a boil and then turn down to a simmer. Continue simmering for about an hour with the pot partially covered. Taste the stock and adjust for salt. After an hour you can turn the soup off until you are ready to eat.
Prepare the fish by cutting filets into 1-2" chunks. Shrimp should be peeled and tails removed but left whole.
About 15 minutes before you are ready to eat, place the prepared fish into the pot and stir gently. Let the fish simmer for about 10 minutes or until it is cooked through. Don't overcook.
To serve, place in deep wide bowls and top with parsley and a generous dollop of spicy Rouille. Enjoy with some warm crusty bread for sopping the wonderful stock.