In a pot of boiling water, blanch the cubed potatoes, and drain. They should be fork tender, but not mushy.
In a heavy-bottomed soup pot or Dutch oven, over medium heat, fry pancetta in the butter until golden. Add the onions and celery to sweat until softened but not coloured. Add the sherry and cook off the alcohol, stirring constantly.
Now add the clams, broth, saffron, and taste for salt. Add pepper and if needed, a bit of salt.
Reduce to low heat, and add the mashed potatoes or dry potato flakes, stirring constantly. Now add the cream, bring up to a simmer, and allow to thicken. You may need to add more potato at this point. You choose how thick or thin you like your chowder.
Add the smoked fish, cubed potatoes, and chives, stir well and serve. Or, you can, at this point, drop in some live clams and mussels, and serve when they open.
Top with garnishes if desired.
Adding charred corn to this soup in late summer is also delicious!