Warm the olive oil in a large heavy pot over medium heat.
Soften onion, leeks, and fennel with a pinch of salt, stirring until translucent.
Stir in garlic and tomato paste. Cook until fragrant and slightly darkened.
Deglaze with white wine. Simmer a couple of minutes to reduce slightly.
Add crushed tomatoes, stock, pastis, orange zest, saffron, thyme, paprika, and red pepper flakes.
Bring to a gentle boil, then add potatoes. Simmer until just tender, about 12–15 minutes.
Season the broth with salt and black pepper to taste.
Add clams and mussels. Cover and cook until they begin to open, about 5 minutes.
Gently nestle in fish and shrimp. Simmer uncovered until fish is opaque, 4–6 minutes.
Discard any unopened clams or mussels. Stir in lemon juice and half the parsley.
Ladle into warm bowls. Top with remaining parsley and serve with crusty bread.