Creamy New England Fish Chowder

Name: Creamy New England Fish Chowder
Ingredients: 1 tbsp butter; 1 tbsp olive oil; 3 large yellow onions – 2 chopped (~2 cups), 1 quartered into layers; 2–3 cups water; 2 bay leaves; Pinch of dried thyme or ¼ tsp fresh thyme; 1 tsp kosher salt (plus more to taste); ¼ tsp black pepper (plus more to taste); 1.5–2 lbs cod (or other white fish — see notes); 8 oz clam juice; 2 lbs potatoes, scrubbed and diced into ½–1 inch cubes; 1 cup whole milk; Small pinch ground nutmeg; ½ tbsp minced chives; 1 cup heavy cream; Optional garnish: extra chives, fresh black pepper, butter, or a sprinkle of nutmeg