Heat butter and olive oil in a large pot over medium heat until melted.
Cook the Onion
Add chopped onion and cook slowly until soft and lightly golden.
Add Celery
Add sliced celery and cook for five minutes, stirring often.
Add Garlic
Add minced garlic and cook for thirty seconds until fragrant.
Add Potatoes
Add diced potatoes to the pot and stir well with vegetables.
Pour Broth
Pour fish stock into the pot. Add salt, pepper, thyme, and bay leaf.
Simmer Gently
Bring to a light boil. Reduce heat and simmer for fifteen minutes.
Add Fish
Add fish chunks gently into the simmering broth.
Cook Fish
Cook for eight minutes until fish becomes tender and flaky.
Add Cream
Pour heavy cream into the pot and stir gently.
Simmer Briefly
Simmer uncovered for five more minutes without boiling.
Finish with Herbs
Remove bay leaf. Add fresh parsley and stir lightly.
Taste and Adjust
Taste the chowder. Adjust salt or pepper before serving.
Notes
Use firm white fish like cod or haddock. Do not boil after adding cream. Cut potatoes evenly for even cooking. Fresh herbs improve overall flavor. Stir gently to keep fish pieces intact.