Melt the butter in a big saucepan with a heavy bottom over medium heat.
Stir in the carrots, celery, onion, leeks, and garlic.
Sauté onions for 4–5 minutes, or until they become transparent.
Mix thoroughly after adding the salt, thyme, and black pepper.
Add the clam juice and fish stock.
Potatoes should be soft after 15 to 18 minutes of simmering under cover.
When the fish is fully cooked, add the cod, peas, and parsley, and simmer for 5 minutes.
Before serving, add the heavy cream, mix thoroughly, and heat for one minute.
If desired, garnish with more parsley.