In a large pot (I recommend a 5-quart pot or larger) over medium heat, cook the bacon, stirring occasionally, until crisp, about 5 minutes. Remove and discard all but 2 tablespoons of the bacon grease from the pot.
Add the onions, celery, carrots, thyme, and parsley to the pot. Cook, stirring occasionally, until the vegetables have begun to soften, about 5 minutes.
Add the garlic and red pepper flakes (if using); cook, stirring constantly, for one minute.
Add the sherry; use a wooden spoon to scrape (deglaze) all the browned bits from the bottom of the pan. Cook for one minute to allow the sherry to reduce a bit.
Stir in the tomato paste; cook, stirring, for one more minute.
Add the clam juice, beef broth, diced tomatoes, and potatoes; increase the heat and bring to a boil.
Reduce the heat and simmer, uncovered, until the potatoes and vegetables are tender, about 15 minutes.
Stir in the clams; gently simmer (don't boil) just until they are heated through (this will keep them from becoming tough and chewy).
Taste and season with salt and pepper as needed.