Use a wide iron skillet. Add the olive oil and the butter together and heat until the butter melts.
Chop the onion and sauté until translucent.
Crush the garlic cloves and sauté another minute.
Chop the celery stalks to small pieces and add. Sauté for 1-2 minutes.
Peel and chop the carrots and the potatoes to small pieces and add. Sauté for 2-3 minutes.
Add a tablespoon of flour if you choose to and mix. The flour will work as a thickening.
If you choose to add the green peas at this stage too. Frozen peas will be fine.
Then add the fish stock and bring to boil. Cook until the vegetables are almost ready. Check the carrots by pocking a thin knife to understand.
Taste and adjust salt, pepper and/or lemon to taste.
Then add the fish gently. Cook - without mixing too much for about 5-7 minutes. Fish cooks fast. Try not to overcook.
Serve with well chopped parsley garnish.