Heat the butter in a pan.
Add the chopped onion and cook gently over medium heat until translucent.
Add the potatoes and thyme, if using.
Pour in the broth. Season with salt and pepper.
Bring to the boil, lower the heat, cover and simmer until the potatoes are very tender, about 15 minutes.
Pour the contents of the pan into the blender and process until smooth.
Pour the puree back into the pan. Bring to the boil.
Add the clams. Cook for a minute. Add more salt and pepper, if needed.
Serve the velvety clam chowder hot.