In a large stockpot or Dutch oven, add clams and cover with water. Cover pot with a lid and cook over medium-high heat until the clams have opened, about 20 minutes. Discard any unopened ones.
Strain the clam broth through a fine-mesh sieve lined with cheesecloth. Set the broth aside. Once the clams are cool enough to handle, remove them from their shells and set them aside.
Rinse the pot that was used to cook the clams. Set over medium heat and add olive oil and bacon. Cook bacon until it starts to brown and the fat has rendered, about 5 to 7 minutes. Transfer bacon to a paper towel-lined plate and set aside. Reserve about 3 tbsp of bacon grease.
To same pot, still set over medium, add onion, carrots, celery, and green bell pepper to the reserved bacon grease. Mix well and cook until softened, but not browned, about 15 minutes.
Add in potatoes and garlic. Continue cooking until potatoes start to soften, about 7 minutes.
Add bay leaf, tomatoes, thyme and reserved clam broth. Stir to combine. Increase heat to high and bring to a boil. Cover and reduce heat to low. Simmer for about 15 minutes.
Chop clams and bacon into small pieces. Add to pot and continue to simmer for an additional 5 minutes.
Remove from heat. Season with salt and black pepper. Remove bay leaf and discard. Garnish with fresh parsley. Serve.