Add gumbo file and paprika to the fish stock, stir, and set aside.
Salt mackerel fillets and shrimp.
Place a large saucepan with olive oil over medium heat until it starts to smoke, then add the fish and shrimp to sear for 3 to 4 minutes. Flip while cooking.
Add the clams, then the octopus a minute later, and cook another 3 to 4 minutes, stirring periodically. Remove the fish fillets and reserve for plating, then pour the fish stock in the pan and lower the heat.
Add butter to thicken the broth and simmer a couple of minutes, then add the chopped herbs, lemon zest and juice, and continue to simmer for 5 minutes, stirring periodically.
Transfer everything to a large bowl.
Toast With Old Bay Aioli
3 Tbsp. Old Bay seasoning
2 cups mayonnaise
2 Tbsp. red wine vinegar
1?2 Tbsp. olive oil
Combine Old Bay, mayo and red wine vinegar, and mix thoroughly.
Sprinkle a little olive oil on sliced sourdough, Italian loaf or similar crusty bread, then grill or toast right before plating.
Serve equal amounts of seafood in soup bowls, add a mackerel fillet in each, and ladle the broth on top.
Spread the aioli mix on the bread, sprinkle a bit of mixed herbs, and serve a slice with every bowl of stew.