Fish chowder

Name: Fish chowder
Ingredients: 1 tablespoon unsalted butter; 1 large yellow onion, chopped; 1 pound cooked fish such as salmon and swordfish; 1 quart whole milk; 2 red bliss potatoes, sliced into eights; 1 stalk of celery, sliced nickel thin; a sprig of fresh thyme; 1 teaspoon Maldon salt flakes or kosher salt, to taste; 1/2 teaspoon black peppercorns, fresh milled; handful of mixed fresh herbs, including parsley and dill