Chop the bacon into 1/2-inch pieces and cook in a large pot over medium heat until crispy, about 8-10 minutes. Remove the bacon with a slotted spoon and set aside on a paper towel. Pour off most of the bacon fat, leaving about 1 tablespoon in the pot. Add the butter and let it melt, then add the diced celery, onion, and bell pepper. Cook over medium heat for 5-7 minutes, stirring occasionally, until the vegetables begin to soften. This creates the flavor foundation for the chowder.
Mince the garlic cloves and add to the softened vegetables, cooking for 30-60 seconds until fragrant. Add the diced potatoes, clam juice, chicken broth, canned clams (with their reserved liquid), black pepper, onion powder, thyme, dill, and Worcestershire sauce. Stir everything together to combine and ensure the seasonings are evenly distributed throughout the liquid.
Bring the pot to a simmer, then cover and reduce heat to medium-low. Simmer for 12-15 minutes, stirring occasionally, until the potato cubes are fork-tender. I like to reserve some cooked potatoes with a firmer texture so the chowder has nice variety in texture rather than becoming a smooth puree.
While the potatoes are simmering, whisk together the cornstarch and heavy cream in a small bowl until no lumps remain. Once the potatoes are tender, slowly pour the cream slurry into the pot while stirring constantly to ensure it's fully incorporated and no lumps form. Add the reserved cooked bacon back into the pot and stir well.
Increase the heat to medium and simmer uncovered for 3-5 minutes, stirring frequently, until the chowder thickens to a creamy, luxurious consistency. The cornstarch will continue to thicken the soup as it heats, so don't overcook it or it may become too thick. Taste and adjust seasonings as needed with additional salt, pepper, or Worcestershire sauce.
While the chowder is simmering and thickening, prepare the bread bowls. Cut the top 1-2 inches off each sourdough bread bowl and set the lids aside. Carefully hollow out the center of each bowl, leaving about 1/2-inch of bread on the sides and bottom to create a sturdy container. I find the bread pieces are great for dunking or serving alongside the chowder.
Ladle the hot clam chowder into each prepared bread bowl, filling it generously but leaving about 1/2-inch of space at the top. Garnish each bowl with fresh chopped parsley and fresh chives. Serve immediately while the bread is still warm and the chowder is steaming hot.