Preheat dutch oven or other large pot over medium-high heat.
Melt butter in the pot. Add onions and garlic. Cook until the onions become soft.
Add flour and stir to make a roux. Stir while it’s cooking so the flour doesn’t burn. Let it get a bit golden.
Add the remaining ingredients except for the fish, shrimp, and cream, and stir to combine. Bring the soup to a boil.
Boil for about a minute and reduce to a simmer. Simmer for about 15 minutes, stirring occasionally. The soup should start to thicken, but if it doesn’t, don’t worry about it – I’ve had a roux (thickening) fail with this soup before because I got distracted and forgot to even make the roux, and it still tasted great.
Turn the heat up to medium and add the fish and shrimp. Cook on medium until fish and shrimp are cooked through, then reduce to low.
Check potatoes and see if they’re done. If not, continue to cook the soup on low until they’re cooked through.
Stir in cream. Taste and add more salt or seafood seasoning if needed.