Preheat oven to 325 degrees. Stir together 3/4 melted butter and flour in a baking dish until smooth. Cover and bake for 30 minutes.

Meanwhile, add remaining butter to a large Dutch oven over medium heat. Add the celery, onion, green peppers and leeks and saute until tender, about 5 minutes. Add the potatoes and clams to the pot, along with enough broth to cover everything. Stir in salt, pepper, thyme and bay leaves.

Bring to a boil, then reduce heat simmer until potatoes are just tender, about 15 minutes. Gradually stir in the baked flour and butter mixture until chowder is very thick. Remove and discard bay leaves, then remove soup from heat and gradually stir in milk or Half and Half to reach desired consistency. Stir in Tabasco sauce and additional salt and pepper before serving, if needed.