How to Make It
Preheat broiler to high with oven rack 4 inches from heat. Brush bread with olive oil, and place on a baking sheet. Broil until golden brown, 3 to 4 minutes, flipping halfway through. Rub toast with 1 garlic clove and keep warm.
Thinly slice remaining 4 garlic cloves. Heat 1/4 cup oil over moderately high heat in a large Dutch oven. Add onion and sliced garlic; cook, stirring occasionally, until translucent, about 5 minutes. Add wine; boil until reduced by half, about 2 minutes. Add tomato sauce and vinegar; bring to a simmer. Add mussels and clams; cover and cook until mussels open, about 5 minutes. Remove mussels with a slotted spoon and place in a large bowl. (Discard any that do not open.) Cover pot and cook until clams open, 3 to 4 minutes. Remove clams with a slotted spoon and place in bowl with mussels.
Season cod, snapper, and shrimp with salt. Add to pot, cover, and reduce heat to moderate; simmer 6 minutes. Add squid, cover, and cook until fish are just cooked through, about 2 minutes. Stir in parsley, mussels, and clams. Remove from heat. Cover and let stand until shellfish are heated through, about 2 minutes. Serve in shallow bowls with a drizzle of olive oil and garlic toast.