Heat olive oil in a saucepan over medium heat. Add onion, celery, carrot, parsley, garlic, bay leaves, and red pepper flakes; cook for 5 minutes until softened.
STEP 2
Stir in mashed tomatoes and cook for 15 minutes. Add fish and vinegar; cook until liquid almost evaporates, about 10 minutes. Add fish stock; cover and simmer for 10 minutes.
STEP 3
Gently stir in clams and cook until opened, about 2 minutes. Add shrimp and cook until pink, about 3 minutes. Season with salt and pepper.
STEP 4
Place a slice of toasted bread in each bowl. Ladle the stew over the bread and serve immediately.