In a small saucepan over medium heat, melt butter. Stir in flour all at once and continue to cook and stir until it again becomes bubbly at the edges. Remove roux from heat and set aside.
In a 10 gallon pot over medium heat, cook onions and celery until just tender. Meanwhile, place milk in a large double boiler and cook slowly over simmering water. Whisk in the roux and cook the milk until thickened, stirring frequently.
Stir clam juice and chicken broth into the large pot and bring to a boil. Then stir in the thickened milk. Bring that just to a simmer, then stir in the heavy cream, thyme and clams. Simmer 10 minutes more, then season with salt and pepper.
Per Serving: 281 calories; protein 28.5g; carbohydrates 11.1g; fat 13g; cholesterol 108.1mg; sodium 352.3mg.