In a small bowl toss shrimp and cod with garlic and 1/2 tablespoon oil and let stand while preparing stew.
In a 3-quart saucepan cook onion and bell pepper in remaining 1 1/2 tablespoons oil over moderate heat, stirring, until softened and beginning to turn golden. Stir in flour and tomato paste and cook over moderately low heat, stirring, 3 minutes. Add broth in a stream, stirring, and simmer, stirring, until mixture is thickened slightly. Stir in tomatoes with reserved juice, Italian seasoning, red pepper flakes, and salt and pepper to taste and simmer 10 minutes.
While stew is simmering, in a 1-quart saucepan combine mussels and vermouth or wine and bring to a broil. Steam mussels, covered, 3 to 5 minutes, or until opened, discarding any unopened ones.
Add shrimp and cod mixture to stew and simmer 5 minutes, or until cod is just cooked through. Add mussels with cooking liquid and parsley and simmer just until mussels are heated through.