Melt the butter in a pan.
Add the chopped bacon to the butter and cook until crisp. Scoop out and set aside.
To the mixture of bacon fat and butter, add the flour all at once. Stir to remove lumps. Throw in the thyme. Cook over medium heat for about five minutes, stirring occasionally, to make a blonde roux.
Pour in 1 and 1/2 cups of clam broth and one cup of milk. Cook, stirring, until thickened.
Stir in one cup of mashed potatoes.
Taste the soup and season with salt and pepper, as needed.
If the soup appears too thick, add more clam broth, milk or mashed potatoes, or all of them. Remember to taste and adjust the seasonings after each addition.
When you're happy with the texture of the soup, add the clams. Simmer for about five minutes.
Sprinkle the bacon and parsley on the clam chowder before serving.