Heat olive oil in a large saucepan, Dutch oven, or clay pot (preferable) over medium heat. Add onion, celery, carrot, parsley, garlic, bay leaves, and pepper flakes; cook and stir until onion has softened and turned translucent, about 5 minutes.

Stir in mashed tomatoes; cook for 15 minutes. Add fish and vinegar (or wine); cook until liquid has almost evaporated, about 10 minutes. Add fish stock, cover, reduce heat to low, and simmer 10 minutes.

Gently stir in clams; cook until opened (discard any clams that don’t open), about 2 minutes. Stir in shrimp; cook until pink, about 3 minutes. Season with salt and black pepper.

Place 1 slice toasted bread in bottom of each bowl. Spoon brodetto over top; serve immediately.