Broth: Wash green and white parts of leek separately in a bowl of cold water, then lift out with a slotted spoon and pat dry. Heat 3 tablespoons oil in a 6-quart heavy pot over moderate heat until hot but not smoking, then cook leek greens, onion, celery, and carrots, stirring, until beginning to soften, about 5 minutes. Add garlic and cook, stirring, 3 minutes. Add saffron, bay leaves, and red pepper flakes and cook, stirring, 2 minutes. Add fresh tomatoes and tomato purée and cook, stirring, 4 minutes. Add wine and simmer, uncovered, until liquid is reduced by about half, about 30 minutes.
Stir in stock and bring to a boil. Reduce heat and simmer, uncovered, 30 minutes. Carefully pour through a fine sieve into a bowl, then discard solids.
Prepare croûtes while broth simmers:
Preheat oven to 300°F.
Heat a well-seasoned ridged grill pan over moderately high heat until hot. Brush bread with remaining 2 tablespoons oil, then grill on all sides until grill marks appear. Tear bread into rough 3-inch pieces and bake on a baking sheet in middle of oven until crisp outside but soft inside, about 10 minutes. Cool croûtes.
Heat butter in a 4-quart heavy saucepan over moderate heat until foam subsides, then cook white part of leek, stirring, until beginning to soften, about 2 minutes. Add fish, hot broth, and salt and bring to a simmer (fish will be cooked through). Remove from heat and stir in 1/2 cup rouille.
Put croûtes in soup plates and ladle soup over. Sprinkle with oregano and parsley and serve remaining rouille on the side.